OK, if you're still confused, here goes. I went through this myself a couple of years ago, so I can relate.
Road (big, external cleats):
• Look = original road clipless pedals. 3-bolt pattern, not walkable.
• SPD SL
= Shimano's version of Look. 3-bolt pattern, not walkable.
• Speedplay Road = double-sided
road pedals, clunkly cleat, not walkable.
I agree with the poster who said you probably don't want these unless all you do is serious road riding/racing and don't really need to walk in your bike shoes.
MTB/Commuting/General Purpose (small, recessed cleats):
• SPD = Shimano's recessed cleat system. 2-bolt pattern. These are cheap and very common. Pedals and shoe options are many. Any "mountain" shoe will work (recessed hole in sole). Lots of companies other than Shimano sell "SPD-compatible" shoes and pedals.
• Eggbeaters/other Crank Brothers = 2-bolt pattern. Same idea as SPD, but different cleats and pedals (don't worry, cleats are always included with pedals). Shoes will be the same as SPD. Some people swear by these. Generally, they are more expensive and less common than SPD. The big thing about Eggbeaters is their 4-sided entry. Widely considered to be the easiest to get in & out of.
• Time Atac = 2-bolt pattern. Just another company's answer to SPD. Proprietary recessed cleats, SPD-compatible shoes. Not sure exactly why anyone would want these.
• Speedplay Frogs = 2-bolt pattern. Speedplay's answer to SPD. Proprietary recessed cleats, SPD-compatible shoes. Their supposed advantage is lots of "float".
See? Not so confusing, is it?
The most important thing is not to confuse SPD with SPD-SL. They are completely different.
FWIW, I went with SPD becasue it's the closest you're going to get to "standard". There are loads of compatible pedals and shoes (Shimano and otherwise, cheap or expensive). I have double-sided on my MTB, platform/clipless combos on my commuter, and single-sided road-specific on my road bike. I use the same pair of shoes on all of them. And, I can walk when I get where I'm going.
If I were to do it all over again, I'd take a hard look at Eggbeaters for the ease of use. But, I'm just too far down the SPD line to switch now...
IMO, full-on road pedals should be avoided for all but the most high-performance road-based applications.