The first letter of the model designates  the engine series:

First character:
G - Servicar three wheeler, 1932 to 1972
E - Overhead valve 61 cubic inch "big twin" (Engine/trans separated)
F - Overhead valve 74 or 80 cubic inch "big twin"
K - Side valve 45 and 55 cubic inch sports bike that replaced the WL in
    1953 and was replaced by the sportster in 1957. It had many design
    features that were carried over to the Sportster.
U - Side valve 74 or 80 cubic inch "big twin"
V - Side valve 74 cubic inch made prior to 1936
W - Side valve 45 cubic inch made 1934 to 1952
X - Sports and special construction. Applied to 1918-1922 opposed twin
    Sport, 1944 military opposed twin, and 1957 to present Sportster.

There were others, such as the J series (a very nice looking bike that was about the hottest
 thing around in the 20's) but you're not likely to see them in daily use anymore.

Second character:

L - high compression. Omitted on low compression early models,
and not indicative of compression on current models.

Remaining characters:

Model descriptions, eg WG - Wide Glide. Some generalizations:

A - Military (Army) version (except GA, Servicar without tow bar)
B - Battery start (early models), Belt drive (some later models).
C - Classic, Competition, Custom, various others meanings.
D - Dyna, the newest frame and engine mount design.
E - Electric start
F - Foot shift (when the standard was hand-shift)
H - varied between High performance and Heavy duty. The early FLH for
    example produced 5 more HP than the FL and was used primarily for
    touring and sidecar applications.
LR- Low Rider (though many Low Riders don't include LR in the model ID)
P - Police version
R - Rubber mounted engine (some models) racing version (other models) e.g.
    the FXR is a lowrider with rubber mounted engine, WLR is the flat
    track racer that was very hard to beat in the 30's, 40's, and 50's.
ST- Soft Tail
S - (without following T) Sports version e.g.  FLHS is an FLHT without the
    touring package.
T - Touring
WG - Wide Glide

Sportsters: Models beginning with X are Sportsters. These descend from the K series,
1952-56. The XL series started in 1957. They are "unit construction"
(engine & transmission share a common case). X was in the normal series,
following the U and V used for flathead Big Twins. L stood for high compression (7.5:1 in 1957).
In 1958 came the XLH with H meaning Higher-power or High-compression (9:1) or Hot.
Also in 1958 was the XLCH: The C in XLCH was intended to mean "Competition",
but the late Hal Robinson used to claim the CH on his bike stood for "Charley Horse"
because it was highly modified and had enough initial ignition
advance to kick back rather severely.

The Sportster engines were originally actually 53.9ci. (883cc) but were designated as 55ci.
This was upgraded (in 1970?) to 61ci (1000cc), redesigned in '73 but still 61" and stayed relatively
unchanged untill the 74" Evo version. The 883 and 61" were both produced the first year of the 883.
Options for Sportsters abounded but these same letters have been used since 1958 in various combinations,
plus the XLCR (a "cafe racer" style, with bikini fairing) and the XLT (Touring, with bigger tank, thicker seat,
and hard bags straight off the FLHS), both produced 1977-1978 only.

Big Twins: Models beginning with F are Big Twins. These all descend from the 61E in 1936.
Knuckleheads 1936-47, Panheads 1948-65, Shovelheads 1966-84/85, and Evolutions 1984/5-present.
The Evolution (block head) was introduced in '84 on the Soft Tail and Tourglide Ultra,
with the last Shovel built in mid '85. Until 1978, these were mostly 74's (~1200cc). In 1978,
the 80ci (~1340cc) was introduced.

In 1971 was the first FX model, with an F engine and assorted Sportster (X) parts,
 notably forks - these were initially Super Glides. In 1974, FXE was Electric start.
In calendar 1977 came the FXS Low Rider, with shortened shocks and scooped seating position.
 Also in 1978, the FX Super Glides got belt final drive. 1979 brought 80ci Shovelheads and the
FXEF Super Glide Fat Bob (not Boy), for bobbed fenders and fat tank. In 1980, FXWG (Wide Glide)
and the FXB Sturgis with belt drive (primary and secondary).

1984 brought the Softtail FXST, with the look of an old hardtail but some suspension for comfort.

In 1991 came the FXD Dyna Glide series, starting with another Sturgis model. This brought many
frame and engine mounting changes, a relocated oil tank and moved crossover pipe.

Touring: In 1980, the FLT designation was introduced for the Tour Glide. This model introduced
 the vibration-isolation mounted engine (rubberglide) and also a slightly different frame from
other FLs and a fixed fairing. FXR (originally called Super Glide II) arrived in late 1981.
The R had rubbermount engine and 5-speed tranny, as opposed to rigid-mount and 4-speed in
 other models. FXRS maybe meant Sport, with cast wheels and a small sissy bar.
The Sport Glide FXRT (Touring) arrived in 1983, with the fixed fairing and air adjustable anti-dive forks.

The only really reliable letters now are at the beginning of the model: X=Sportster, F=Big Twin,
FL=Big Twin with big forks, FX=Big Twin with Sportster-style (smaller) forks, FXST=Softtail,
FXD=Dyna Glide. At times, different models have even shared the same letter-designation;
 witness the FLHTC Electra Glide Classic/Ultra Classic Electra Glide in 1993 models.

Designations

THE BASICS FROM THE PAST
     E          the basic version of the big twin
     EL        the E with more power
     F          basic, enlarged from 61 to 74 big twin
     FL        the F with more power
     FLH     still more power built as pan shovel and evo
     K          side-valve middleweight circa '52
     KK        tuned version of K
     KH        the K stroked from 45 to 54
     KHK     tuned version of the KH
     XL         basic sportster
     XLC      stripped Calif. model
     XLH      higher performance XL '58
     XLCH    high performance XL (w/ kicker)
     FX         original Super Glide form F and X parts
     FXE      original Super Glide form F and X parts with electric start

SPORTSTERS
     XLR           hot stripped XL
     XR-750     iron and alloy 750 racing only
     XLCR        road race style XLH
     XR-1000   alloy XR top end and dual carbs on the XLH cases
     XLT           XLH with larger tank
     XLX          stripped XLH
     XLS           fancy XLH
     XLH          most current Sporty 883 and 1200
     XL            '96 1200C (Custom) and 1200S (sport) Sportsters  1200R (Roadster)

LOW RIDERS
     FXS            the original Low Rider
     FXEF          the first Fat Bob F=Fat
     FXWG        FL forks w/o covers and 21" front wheel aka Wide Glide
     FXB            original Sturgis B=belt drive
     FXR            Super Glide II original rubber mount
     FXRP         P=police equip.
     FXRS         FXR with cast wheels, extra trim a.k.a.  Low Glide
     FXSB         Low Rider with belts
     FXRDG       FXRS with solid disk rear wheel
     FXRT          R=Rubber mount T=touring a.k.a. Sport Glide
     FXRC         Low Glide Custom w/ wire wheels
     FXRT/P      FXRT P=police equip.
     FXRD         FXRT w/ trunk aka Grand Touring
     FXRS-SP     Low Rider Sport edition
     FXRS-CONV   Low Rider w/ detachable windshield and bags
     FXDB         Dyna Glide, latest chassis ('91 Sturgis) two-point engine mounting
     FXDC         FXDB with goodies C=Custom (NO 21" front wheel)
     FXD            Dyna Super Glide
     FXDS-CONV   Dyna Glide w/ detachable windshield and bags
     FXDWG       Dyna Wide Glide
     FXDL          Dyna Low Rider
     FXDX         Dyna Super Glide Sport
     FXDC         Dyba Super Glide Custom

SOFTAILS
     FXST          Original Softail
     FXSTC       Softail Custom
     FXSTD       Softail Deuce
     FXSTS       Springer Softail
     FXSTSB    Springer Softail Badboy
     FXSTB       Night Train
     FLST          Softail with 16" front wheel
     FLSTC       Heritage Softail
     FLSTF        Fatboy
     FLSTN        Fatboy/Heritage with different goodies (N for Nostalgia)
     FLSTN        2005  Softail Deluxe
     FLSTFSE   Screamin' Eagle Fat Boy
    FLSTSC      Softail Springer Classic
     

ELECTRA GLIDES
     FLT            Tour Glide, has frame-mounted fairing
     FLTC          FLT with extras (C for Classic)
     FLTC U      FLT with even more extras (U for Ultra)
     FLTC U I    FLTC U with fuel injection (I for Injection)

     FLHT            Electra Glide, has fork-mounted fairing
     FLHS            FLT with windshield and less goodies S = Sport
     FLHTC          FLHT with extras (C for Classic)
     FLTRI             Road Glide          
     FLHTCSE     Screamin' Eagle Electra Glide 2
     FLHTC U       FLHT with even more extras (U for Ultra)
     FLHTC U I      FLTC U with fuel injection (I for Injection)
     FLHR             Road King, factory customized FLHT
     FLHRS           Road King Custom
     FLHRCI          Road King Classic

V-RODS

    VRSCA           V-Rod
    VRSCB           V-Rod  no extra stuff
    VRSCSE         Screamin' Eagle V-Rod