I have hard so many different things and rumors
I know, you just wanted to start an argument and then

opcorn: right?
steveo covered the basics.
LT1 was a relatively short-lived platform, '92-'97, although based on the venerable small block Chevy introduced in 1955. It was produced in both 2-bolt and 4-bolt main bearing cap versions. It applied sequential electronic port injection for the first time in a GM V8, used "reverse cooling" that routed coolant through the heads first for better combustion chamber temperature control, had improved chamber design, and moved the distributor to the front of the engine. That distributor move, Optispark, is probably the most common complaint about the engine due to susceptibility to water (later fixed).
While the LT1 was in production, GM started with a blank sheet of paper for the LS1. In the end, the only things carried over from the SBC and LT1/4 was the connecting rod bearing size, and roller lifters (which were also later improved). The block was beefed up with 6 main bearing cap bolts standard, the cam was moved up in the block, the heads were completely different (and very efficient), a crank-triggered ignition with single coil per cylinder was implemented, connecting rods lengthened (from the typical 5.700" SBC to 6.098"). The bore-to-stroke in the original LS1/6 was "under-square", meaning the stroke was longer than the bore diameter, a departure from SBC and BBC. Rocker arms have needle bearing folcrums and are very light weight but strong. The bellhousing pattern is basically the same (moved a bolt from behind the right cylinder bank to the top center), but lower bellhousing bolts to the oil pan were incorporated (oh, the oil pan is cast and a structural member of the already beefier block). Originally designed as an aluminum block with cast-in steel cylinder liners, it was later cast in iron for truck engines, which improved the strength even more. The side engine mounts are moved relative to the bell housing vs. the SBC and BBC location. The intake manifold is molded nylon vs. metallic.
With only 6 years in production for the LT1/4, and limited to Corvettes, Camaros, and some full-size RWD passenger cars, the aftermarket support is somewhat limited. The LS engines, on the other hand, have been in production for 16 years now, and have been used in sports cars, trucks, SUVs, and even FWD cars. Aftermarket support is extensive. You'll probably hear complaints about the higher cost of LS engines, but $'s/HP, they are very competitive.
But, even with GM parts, they can put out some impressive power. I've got an LS1 in an '82 Camaro that is all-GM (truck heads were shaved to bring the chambers down to LS1 size) except for ARP rod and head bolts, and Melling oil pump, with headers and a free-flowing air inlet system. The pistons have never been out of the bores. Factory bearings, push rods, rockers, valve springs, cam, ignition. With a higher stall converter, 3.73 gears, and sticky tires, on a stock tune, it pushes the 3400 lb car through the quarter mile in 11.9-12.0 seconds. Not too shabby for a car I drive to work year-round.
Having said all that, like steveo said, the LT1 is a nice engine.
I guess we should have asked - Why are you asking?