There's a LOT.
Tiny races gain... a +2 size bonus on attack rolls
Tiny characters take up a space of 2-1/2 feet by 2-1/2 feet, so up to four of these characters can fit into a single square. Tiny races typically have a natural reach of 0 feet, meaning they can't reach into adjacent squares. They must enter an opponent's square to attack it in melee. This provokes an attack of opportunity from the opponent. Since they have no natural reach, they do not threaten the squares around them. Other creatures can move through those squares without provoking attacks of opportunity. Tiny creatures typically cannot flank an enemy.
Creatures that are size Tiny or smaller use their Dexterity modifier in place of their Strength modifier to determine their CMB. The special size modifier for a creature’s Combat Maneuver Bonus is as follows:
Fine –8, Diminutive –4, Tiny –2
Tiny manufactured weapons also deal less damage than their medium counterparts, according to Table: Tiny and Large Weapon Damage. Some Tiny weapons even deal '--' damage. Tiny creatures usually receive no bonus from using reach weapons, as they double the user's natural reach (in this case 0 feet most of the time).
Tiny and Smaller weapons are priceless and weightless-- the prices listed in the CRB are for medium weapons and no rules are given for the calculation of weapons smaller than small or larger than large. Generally people just use the armor rules for this too, but that 1/2 off magic items multiplier is a bit severe. YMMV.
The armor for unusual creatures table shows that armor for tiny or smaller creatures offers only half the usual bonus.
Tiny or smaller*
*Divide armor bonus by 2.
but they also receive a +2 size modifier to AC:
You receive a bonus or penalty to your AC based on your size...
The table wind effects shows the size of creatures that need to make a check or that are blown away. This starts earlier for tiny or smaller sized creatures.
The special size modifier for a creature’s Combat Maneuver Defense is as follows:
Fine –8, Diminutive –4, Tiny –2
Armor for tiny creatures (including magical armor) is cheaper and lighter than that of larger creatures. The price of such armors is modified by:
Tiny or smaller humanoid cost ×1/2 weight ×1/10
Tiny or smaller nonhumanoid cost ×1 weight ×1/10
The rules are unclear as to whether or not and if so how fast you starve, suffocate, or dehydrate:
Nonlethal damage from thirst or starvation cannot be recovered until the character gets food or water, as needed.
but 'as needed' is only defined for medium and small creatures.
Suffocation is fine as long as it's not slow suffocation in a sealed 10'X10'X10' chamber which has its own special rules that neglect to mention the air consumption rate of tiny creatures.
The tracks of tiny creatures are harder to detect and the survival DC to follow them is modified by the table entry:
Very Small Creature: A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.
Size: A creature larger or smaller than Medium takes a size bonus or penalty on Fly checks depending on its size category:
Size: Creatures gain a bonus or penalty on Stealth checks based on their size: Fine +16, Diminutive +12, Tiny +8
A smaller creature can carry less weight depending on its size category, as follows: Small ×3/4, Tiny ×1/2, Diminutive ×1/4, Fine ×1/8.
Quadrupeds can carry heavier loads than bipeds can. Multiply the values corresponding to the creature’s Strength score from Table: Carrying Capacity by the appropriate modifier, as follows: Fine ×1/4, Diminutive ×1/2, Tiny ×3/4,
Certain items of equipment, like waterskins, do not exist in sizes designed for Tiny creatures. Most non-combat items don't have specified alterations based on price, which means the 'medium' version is sort of the only one available unless you can get something out of your DM. You do start with one set of clothes of your choice for free, however.
Some magic items, such as Death's Head Coffers and Featherleaf Barding, provide additional potential benefits to tiny creatures.
Tiny animals get to use Dex instead of Str for Climb and Swim.
Small(sic) animals like these use Dexterity to modify Climb and Swim checks.
The above quote refers to familiars, which are usually tiny animals, not small ones.
Magic in general, and any other subsystems using fixed measurements regardless of size, is much more potent from the perspective of Tiny creatures.
Additional rule systems, such as those for mass combat and race creation, also reference creature size and make special rules and accommodations for tiny creatures. Depending on what additional rule systems you play with, there may be even more changes.