Simple

I ate pizza yesterday.

To indicate a past habit – or an action already completed.
Can be used with or without adverbs of time.

I eat pizza everyday.

To express habits or general truth.
To indicate a future event on a designated date as part of a plan or arrangement.
With 'mental action' verbs:
like, love, want, need, believe, etc

I will eat pizza tomorrow.

To indicate an action, condition, or circumstance which hasn't taken place yet.

Continuous

I was eating pizza when you arrived.

To indicate uncompleted action of the past (with or without time reference)

To indicate persistent habits of the past (with always,
continuously, forever, etc.

I am eating pizza right now.

To indicate action going on at the time of speaking.
To indicate temporary action which may not be happening at the time of speaking.
With a habitual action verb, especially to indicate a stubborn habit.

I will be eating pizza when you arrive.

To indicate what will be going on at some time in the future.
To indicate planned future events.

Perfect

I had eaten all of the pizza when you arrived.

To indicate a completed action of the past that happened before another event took place.

I have eaten all of the pizza.

To indicate past action which is not defined by a time of occurrence.
To indicate an action which started in the past and has continued up until now.

I will have eaten all of the pizza by the time you arrive.

To indicate an action that will be complete before another event takes place.

Perfect
Continuous

I had been eating pizza for 2 hours when you arrived.

To indicate an action in the past that began before a certain point in the past and continued up until that time.

I have been eating pizza for 2 hours.

To indicate an action which started at some point in the past and may or may not be complete.

I will have been eating pizza for 2 hours when you arrive.

To indicate an action that will have happened for some time and will not be complete yet at a certain point in the future.