3e is the default we all know. There is tons of material out for it (lots and lots of modules).
Pathfinder is 3e with a bit of a power bump to core classes and races to make them compatible power wise with later 3.5 introduced base classes and races which had some power creep (beguiler, Book of Nine Swords classes, warforged race, etc.) Pathfinder also simplified skills, grappling, turned save or die spells into damage effects and changed polymorph and wildshape to be less variable depending on sources used. Can easily use 3e resources, particularly modules.
4e takes the 3e ideal of each class being roughly balanced for combat encounters and makes it more uniform with each class having the same resource management of at will, encounter, and daily power class ability structure. It reduces 3e multiclassing to single class dipping which reduces poor multiclass builds. Monsters and NPCs have smaller statblocks and are easier to create from scratch. Some combat rule changes to make things run smoother (duration need not be counted down but save rolled each round, monsters have fewer parts to track, no real dispel magic causing recalculation of bonuses, etc.) More hp than 3e but no full attack options. More movement based powers for characters. Skills are consolidated and there are skill challenge mechanics (with criticisms about the math involved). Few current resources (modules in particular, but also classes and charm, summoning, and necromantic magic).
For casual play 4e has great options for the DM with easy to create monsters and NPCs, easy to use monster statblocks with a few distinctive powers each. On the downside there are not many module options and conversions have a few math issues. Also there is no easy to reference online srd such as
www.d20srd.org.
For players all classes have resource management of at will, encounter, and daily powers plus action points, but character building to balanced characters is easier. Group tactics are more important. Requires learning the new system.
Pathfinder is relearning the 3e classes again with added powers and tweaked spells but looks to smooth out some of the perceived issues of 3e (save or die, power creep, empty levels, skill characters being spread thin by related skills). It allows use of existing 3e materials and there are a ton of 3e modules and settings and splat books out there available.
3e allows builds that do variable amounts of resource management.
You can have a warlock who has all ongoing or at will magic abilities. A rogue who just has to figure out how to get into position to sneak attack in combat and not worry about 1/day and 1/encounter abilities. Fighters who hack things well all the time.