LS1=346ci 5.7L started in 97 in the Vettes, iron sleeved Aluminum block, 3.89" bore, 3.622" stroke, all metric complete redesign of the small block chevy, used a pair of knock sensors in the valley, 1x cam sensor, 24x crank sensor, 6 bolt mains, powdered metal connecting rods, hyper pistonsm, 853 casting heads early production, 241 casting heads by 2001
LS6= same as an LS1 with better cam, better 243 casting heads with hollow/sodium filled valves, better flowing intake manifold, updated casting process for the block making it somewhat more desireable then a standard LS1
LS2=368ci 6.0L started life in the 2004 Vettes, 4.0" bore 3.622" stroke, better casting stronger block, moved knock sensors, moved cam sensor, 58x reluctor wheel for the crank sensor, used the same 243 casting heads from the LS6 without the hollow and sodium filled valves, 90mm throttle body, revised intake
LS3= 376ci 6.2L, started life in 2008 Vettes, completely revised heads with rectangular intake ports as opposed to the older cathedral design, offset intake rockers, 4.068" bore 3.622" stroke, 4x cam signal
LS4= 5.3L waste of time came in FWD cars and doesn't share as much with the RWD engines, first offering of displacement on demand (which is junk)
LS7= 427ci 7.0L first offered in the 2006 Corvette Z06, 4.125" bore, 4.0" stroke, block is very similar to the other LS engines but it stops there, Unique heads to the LS7 outflowing anything else factory, intake, cam, valvetrain, crankshaft has a longer snout to accomadate the dry sump oiling system, better flowing exhaust manifolds, this engine is very unique and is hand assembled
LS9= 376ci 6.2L, available only in the ZR1 Corvette, shares a lot of the design with the LS3, but has the dry sump stuff from an LS7, low compression for the 3 rotor eaton blower, more exotic materials throughout,
LSA= more toned down version of the LS9, only offered in the Cadillac CTS-V, basically a lower compression LS3 with a supercharger
LSx is either the GMPP block that is available as a standard or tall deck, or its a designation for all things LS
There are truck versions of the LS1, in a 4.8L, 5.3L, and 6.0L made of iron. There are other oddball variants floating around, like the aluminum 5.3L that came in certain trailblazers and a couple other things. The L92 aluminum 6.2L truck engine thats very similar to the LS3. There are at least a dozen other RPO codes for various truck engines, a couple car engines that are available in Europe and Australia, whatever the code is for the new automatic Camaros which is basically an LS3 with active fuel management (displacement on demand).
Okay I'm done, I didn't look any of that up, seriously, that is all off the top of my head.