Reiatsu (better known as Power Level bleach-style) is the total measure of your 'power' as a character. As this rises, it is distributed among your stats. (Yes I'm aware this is screwing with bleach's core terminology, but this is less confusing than canon xD)

Strength: your physical power, ability to apply force, etc. Obviously being bleach, there will be characters who screw physics, but we request that in general you try to line up your strength with your character's mass. A child sized character will usually be significantly weaker (relative to their total Reiatsu) than an adult sized character, who will usually be significantly weaker (relative to their total Reiatsu) than a giant sized character. Also in roleplay try to take mass into consideration, if a character is twice your size, he's going to seem a little stronger than his stats indicate (how much is up to both participants. Be reasonable.)

Defense: this represents your character's defensive ability. It's a combination of many things, your instincts, ability to roll with the blows, your physical toughness, etc etc. The higher your defense, the harder you are to hurt or negatively affect.

Speed: This is your character's physical speed. It determines how fast you attack, defend, etc. Bear in mind that this site has taken measures to keep speed 'in line' with the value of other stats, so it's not as valuable as in many rp's.

Spirit: This measures how strong your soul is, how much energy you are able to command during any given turn. Contrary to some games, this stat is NOT spent, but instead is simply a measure of your spiritual strength.

These four basic stats are then used to derive multiple substats  (click here for further information) which more specifically affect your character's ability in combat. When making stat/substat comparisons, this chart is a rough guideline of how they interact.