First of all, all pathfinder material (even from third-party publishers) is available in the SRD. That's a huge bonus right there.
This thread says a bit about the differences between 3.5 and Pathfinder.
The most notable ones (to me):
General changes:
- No more cross-class skill penalty (having to pay twice for the same amount of skill). Many skills got grouped together (Perception replaces Spot, Listen and Search, Acrobatics replaces Balance and Tumble etc.)
- Feats at every odd-numbered level, resulting in more feats (and thus more customization).
- Concentration is no longer a skill, and rather a caster level + ability score check, and thus much harder.
- Nothing costs XP anymore, and you can't permanently loose levels or ability scores due to anything.
- No more XP-penalty for multiclassing. Instead you get to choose one favored class, and for each level you take in it you get +1 HP, +1 skill point or something else depending on your race/class.
- Power attack is now more limited by your BAB. Ranged combatants get Deadly Aim, which works like Power Attack for ranged weapons (and is a huge boon for them). Arcane Casters get Arcane Strike, which trades a swift action for a minor damage boost based on caster level.
Old Classes:
All classes get a large amount of archetypes - those swap out a set of class features for something else. That allows great customization.
- Barbarians get Rage powers - special effects that only work while raging. However, rage is now measured in rounds/day rather than activations/day, which is generally a nerf.
- Bards get slightly better spells, but bardic music is now measured in rounds instead of activations. The archetypes are nice, but generally bards are worse off mostly due to the bardic music change.
- Clerics no longer start out with heavy armor proficiency. Turn Undead is now Channel Energy, which has more uses (including healing)
- Druids can now pick domains instead of an animal companion. The latter got nerfed a bit. Shapeshifting, and therefore Wild Shape, now just gives a bonus to attributes and extra abilities instead of replacing your attributes - a much needed change, making druid balance much better.
- Fighters get class features that improve armor and weapons. The most notable is that they can move at full speed in heavy armor.
- Monks are now actually a decent class. They get Ki, which is a limited daily resource that can be spent for various effects. More importantly, Flurry of Blows now works much better - for the purpose of using it (and combat maneuvers), monks are effectively a full-BAB class.
- Paladins no longer need Wisdom. Smite Evil is now a great ability that buffs you for the entire encounter, rather than working on a single strike. You can also forgo your mount and forge a divine bond with a weapon instead.
- Rangers get some new class features and their animal companion scales better.
- Rogues now have their sneak attack work on (almost) everything. They get Rogue Talents from level 2 at every second level, which allow huge variety. However, everyone can now find and disarm all non-magical traps, but Rogues are still better at it.
- Sorcerers now get to choose one bloodline (arcane, draconic, shadow and much more) that grants them extra feats, extra spells known and extra class features.
- Wizards now get powers based on the school they specialize in.
New Classes:
Gish-classes are spellcasters that only get 6-th level spells and have medium BAB. The Bard is one as well.
- Alchemists are an arcane intelligence Gish-class caster class that work their magic via potions and magic bombs.
- Cavaliers are mounted leaders and chargers. They get a challenge ability that is similar to the marks of D&D 4
- Gunslingers use gunpower weapons. The way they are written actually allows you to use them without giving such weapons to anyone else.
- Inquisitors are a divine wisdom Gish-Class mix in some skill-monkey abilities.
- The Magus is an arcane intelligence Gish-class that focusses on casting spells in melee while wielding a weapon in one hand and a spell in the other. They gain a daily arcana pool which can be used for various abilities, such as remembering an already cast spell. They can also cast in light and eventually in medium and heavy armor.
- Oracles are full divine spontaneous charisma-casters. They gain a specific weakness (such as impeded sight or movement) for a large array of boni.
- The Summoner is a full arcane charisma caster that focuses (surprise!) on summoning. They get Summon Monster as a spell-like ability, but their notable features is their Eidolon: a customizable creature that they can have active at all times.
- The Witch is a full arcane intelligence caster that mostly focusses on debuffing via her hexes.
Archetypes:
As mentioned, those provide a huge amount of variety.
For example, a Bard could be an Archaeologist and trade Bardic Music for trapfinding and rogue talents, or an Arcane Duelist and eventually learn to cast in medium and heavy armor. A Barbarian can trade his mobility (fast movement, uncanny dodge) and learn how to use heavy armor particularly well.
Each class has about a dozen options for archetypes, and as a long as they don't affect the same class feature you can take multiple archetypes.