Developer
Developer is a chemical combined with certain products to ensure they work correctly. The main ingredient in this product is hydrogen peroxide, which correspondes to the level or number developer.
Hydrogen Peroxide lifts the first level of your hair (the cuticle) and allows product to penetrate the second layer (the cortex), where the color lies.
Developer comes in volumes (vol), usually represented in increments of 10, 20, 30, 40 and 50.
This essentially translates to how much of your cuticle will be lifted. 10 volume, being a lower volume, would only lift the cuticle slightly, and deposit the color. As you go up in volume, the effects of the chemical reaction increases and your hair is lifted higher. 20 vol lifts the hair 2 levels, 30 vol 3 level, 40 vol 4 levels, 50 vol 5 levels.
You may also mix developers to create a stronger volume. Say you need a 30 vol for your project and the supply store only has 10 vol and 20 vol, you can combine those to get your desired 30 vol.
You mix developer with both bleach and certain hair dyes to acheive different outcomes. The developer is the crucial ingredient to recieving your desired outcome.
The most commonly used volumes are 10 - 30.