A sentence is a complete thought, containing both subject and verb. The subject is what the sentence is about, and the verb is what the subject is doing.
Here’s an example of a sentence: I write.
“I” is the subject, and “write” is what I do.
Simple as that. (“Simple as that” is not a complete thought; it’s a sentence fragment, the sort of thing your English teacher would murder with red ink, but which creative writers can get away with when they know what they’re doing. “Simple as that” has neither a subject nor a verb. It’s a modifying phrase, and phrases aren’t sentences at all; they’re not even clauses.)
Sentences come in several varieties determined by the kind and number of the clauses it contains.
A clause is either independent, containing both a subject and verb of its own, or dependent, meaning it has to lean on…