Polymorph

Taking a shape here does not take your gear away: what you are wearing is merged into the shape.

What comes with you

Your weapon, or your gloves if you have no weapon. With a weapon equipped, bracers are merged too.
Your off hand, except its damage properties. A shield's properties are merged.
Ammunition, only if it is unlimited, and only while you hold the bow it belongs to.
Armour class from your items, but only the best single value, counted as armour.
Ability, skill and saving throw bonuses, which stack up to their usual ceilings.

Shapes give no hit points of their own.

What changes about you

Every shape adds deflection armour class, by shape. Tenser's Transformation and Shapechange have their own.
Monk abilities stop working while you are shifted - the attack scaling, the armour class, the speed, all of it.
Spell Focus in Transmutation improves every shape: +1 attack and deflection for the feat, +2 for the greater version, +4 for the epic one.
A Shifter of level 30 gets better shifter shapes and one extra attack a round.
You cannot level up while shifted.

Casting from a shape

Every shape has at least one ability of its own, and the save DC does not come from the ordinary rule - see Magic for both formulas: a druid or shifter shape uses your druid and shifter levels with Wisdom, any other shape uses the ability's own level with your casting ability and Spell Focus.

Every shape

The full list - all 228 forms the game will actually put you in, with their statistics and the spell that grants each - is on Shapes you can take. It is built from the module's own table, so it cannot fall behind the game.