Size: Light, Medium, Heavy. Each requires a proficiency to use. For example, Wizards get no armor proficiency, Rogues get Light armor proficiency, and Fighters get Light, Medium, and Heavy armor proficiencies. Warforged are automatically proficient with any of their body types.
Maximum Dex Bonus: This limits how much of your Dexterity bonus you can apply to your AC. If you have a 17 Dexterity (+3 bonus) and are wearing Full Plate, which has a +1 Maximum Dex Bonus, then you can only apply +1 out of your +3 to your AC. You'll still get your full Dexterity bonus to other things, like your Reflex save and skills. So there is a balancing act to maintain there between the Armor Bonus of the armor, and the Maximum Dex Bonus of the armor. Mithral armor (normal mithral armor, not talking about Warforged yet) will raise the Max Dex Bonus by 2, and there are some Fighter and Dwarf Enhancements that can raise it too.
Armor Check Penalty: This is a penalty that gets applied to certain skills: Balance, Jump, Hide, Move Silently, and Tumble. The Armor Check Penalty is applied double to your Swim skill. Heavier armors have more of a penalty.
Arcane Spell Failure: This is a percent chance that an Arcane spell caster has to fail a spell they cast. That means Bards (unless they're in Light armor), Wizards, and Sorcerers. Paladins and Clerics can cast their spells in any armor without a problem. There are some equipment properties to lower this number, and Drow and Elves have some Enhancements to help there too.
So I guess that's the broad overview of armor.
For Warforged it's just slightly different since their armor is 'built in'.
Base Composite Plating, is Light Armor. It provides a +2 Armor Bonus and 5% Arcane Spell Failure. Any class can play a Warforged just fine in that base Composite Plating. Arcane spellcasters might want to use an item or a Warforged Enhancement (Inscribed Armor) to make that 5% chance of spell failure go away.
Mithral Body, it also counts as Light Armor. It provides a +5 Armor Bonus, 15% Arcane Spell Failure, -2 Armor Check Penalty, and a +5 Maximum Dex Bonus. Again, most classes would be fine in this, though the Arcane spellcasters might want to invest more resources to get rid of that 15% chance of failing a spell. Also, if you plan on having a high Dexterity, that +5 limit on it may actually end up lowering your AC at some point. Some people may not like the -2 to some skills, but that's not really too bad. There are Feats to raise the Max Dex Bonus, and you can take them more than once--so you could get the Max Dex on this armor up to +10 if you spent your Feats at levels 3, 6, 9, 12, and 15 on it.
Adamantine Body, it's Heavy Armor. It provides +8 Armor Bonus, 35% Arcane Spell Failure, -5 Armor Check Penalty, +1 Max Dex Bonus, and Damage Reduction 2 that can be bypassed with Adamantine weapons (which are fairly uncommon, really). Most Arcane spellcasters probably won't be able to overcome that 35% Arcane Spell Failure, but it IS possible with some high-end gear (and the Inscribed Armor Enhancements). The very low Max Dex Bonus may make you actually lose AC if you have a very high Dex. This body is particularly good for your low-dex thuggish smashy types. Also, you can't use Evasion in Heavy Armor, so for example, a Monk, Rogue or Ranger in Adamantine Body may not be a good idea (unless you don't care about Evasion).
There are Enhancements you can take to get more DR in any of the body types.
Some people just can't afford to take a Body Feat since they may need that Feat choice for something more important. Also, you CAN respec out your body Feat if it's not working out for you, or if you want to upgrade later.
Maximum Dex Bonus: This limits how much of your Dexterity bonus you can apply to your AC. If you have a 17 Dexterity (+3 bonus) and are wearing Full Plate, which has a +1 Maximum Dex Bonus, then you can only apply +1 out of your +3 to your AC. You'll still get your full Dexterity bonus to other things, like your Reflex save and skills. So there is a balancing act to maintain there between the Armor Bonus of the armor, and the Maximum Dex Bonus of the armor. Mithral armor (normal mithral armor, not talking about Warforged yet) will raise the Max Dex Bonus by 2, and there are some Fighter and Dwarf Enhancements that can raise it too.
Armor Check Penalty: This is a penalty that gets applied to certain skills: Balance, Jump, Hide, Move Silently, and Tumble. The Armor Check Penalty is applied double to your Swim skill. Heavier armors have more of a penalty.
Arcane Spell Failure: This is a percent chance that an Arcane spell caster has to fail a spell they cast. That means Bards (unless they're in Light armor), Wizards, and Sorcerers. Paladins and Clerics can cast their spells in any armor without a problem. There are some equipment properties to lower this number, and Drow and Elves have some Enhancements to help there too.
So I guess that's the broad overview of armor.
For Warforged it's just slightly different since their armor is 'built in'.
Base Composite Plating, is Light Armor. It provides a +2 Armor Bonus and 5% Arcane Spell Failure. Any class can play a Warforged just fine in that base Composite Plating. Arcane spellcasters might want to use an item or a Warforged Enhancement (Inscribed Armor) to make that 5% chance of spell failure go away.
Mithral Body, it also counts as Light Armor. It provides a +5 Armor Bonus, 15% Arcane Spell Failure, -2 Armor Check Penalty, and a +5 Maximum Dex Bonus. Again, most classes would be fine in this, though the Arcane spellcasters might want to invest more resources to get rid of that 15% chance of failing a spell. Also, if you plan on having a high Dexterity, that +5 limit on it may actually end up lowering your AC at some point. Some people may not like the -2 to some skills, but that's not really too bad. There are Feats to raise the Max Dex Bonus, and you can take them more than once--so you could get the Max Dex on this armor up to +10 if you spent your Feats at levels 3, 6, 9, 12, and 15 on it.
Adamantine Body, it's Heavy Armor. It provides +8 Armor Bonus, 35% Arcane Spell Failure, -5 Armor Check Penalty, +1 Max Dex Bonus, and Damage Reduction 2 that can be bypassed with Adamantine weapons (which are fairly uncommon, really). Most Arcane spellcasters probably won't be able to overcome that 35% Arcane Spell Failure, but it IS possible with some high-end gear (and the Inscribed Armor Enhancements). The very low Max Dex Bonus may make you actually lose AC if you have a very high Dex. This body is particularly good for your low-dex thuggish smashy types. Also, you can't use Evasion in Heavy Armor, so for example, a Monk, Rogue or Ranger in Adamantine Body may not be a good idea (unless you don't care about Evasion).
There are Enhancements you can take to get more DR in any of the body types.
Some people just can't afford to take a Body Feat since they may need that Feat choice for something more important. Also, you CAN respec out your body Feat if it's not working out for you, or if you want to upgrade later.