A schema is a plan or an outline that is also a diagram. It is pronounced (skee ma). The plural may be made by either schemas or schemata. Note, that schemata, while uncommon, is often misused as a singular form, but it is in fact plural.
There are many derivatives of schema, including schematic. Schematic, pronounced (skee maa tick), may be used as an adjective or noun. Because schema dates from around 1700 and schematic from the 1930s, our guess is that the noun form of schematic came from the pervasive misuse of the adjective. That is pure speculation on our part, based on how words tend to change over time.
The adverb form is schematically, and it is usually used when referring to the schematic or plan of events.
Scheme comes from the same root, and can be used to mean a plan or outline, but the more common definition is a plan in which to gain something by deception or thievery.