Zaltias

1. It depends. Lego Star Wars 1 has all the characters from the 1st 3 movies. It has less features than Lego Star Wars2 2. In the second one, you can build stuff from lego bricks, different force powers, you can make a custom character etc. In both, you can either be Jedi, Droid support or a Gunner. Jedi have lightsabers and use the force to manipulate things in the game e.g Jedi mind trick, Force push. Droids help you open doors and are otherwise useless (Battle droids, Destroyer Droids (LSW1) and IG-88 (LSW2) are combat droids and have different purposes)
Gunners have ranged weapon. They also have grappling hooks to allow them to access places that most characters cannot reach. In LSW2, Bounty hunters are introduced. Thy have the Thermal Detonators, which OHKOs most enemies. Also, in LSW2 there are more features than the original game.

2. Yes, LSW1 is Episode 1, 2 and 3. While LSW2 is episode 4, 5 and 6.

3. Some of the bosses from both games include - Darth Maul, Count Dooku, General Grievous, Anakin, Death star space battle, Darth Vadar, The Emperor.

Hope all this helps =D

Zaltias