A SIMPLE SENTENCE shows one idea.
Cherokee is a great school!
A COMPOUND SENTENCE combines two ideas using a comma and a conjunction. Think FANBOYS (for, and, nor, but, or, yet, so)
Cherokee is a great school, so I always look forward to going there.
A COMPLEX SENTENCE has an INDEPENDENT CLAUSE joined by a DEPENDENT CLAUSE. Dependent Clauses begin with a conjunction and can have a comma. (Signal Words: As, After, Although, When, While, Until, Because, If, Since)
While I was at Cherokee, I learned so many things.