A sentence is the largest unit of any language. In English, it begins with a capital letter and ends with a full-stop, or a question mark, or an exclamation mark.
Example:
She is a good girl (statement), Is she a good girl? (question), What a nice weather! (exclaiming).

Ideally, a sentence requires at least one subject and one verb. Sometimes the subject of a sentence can be hidden, but the verb must be visible and present in the sentence. Verb is called the heart of a sentence.  
Example:
Do it. (In this sentence, a subject ‘you’ is hidden but verb ‘do’ is visible)   
Types of Sentences:
Structurally, sentences are of four types:

Simple sentence
Compound sentence
Complex sentence, and
Compound-complex sentence.

Simple sentence

A simple sentence must have a single clause (a single verb) which is independent, and it cannot take another clause.
Example:
I always wanted to become a singer. (One clause – one verb)

Compound sentence
A compound sentence must have more than one independent clause with no dependent clauses. Some specific conjunctions, punctuation, or both are used to join together these clauses.
Example:
I always wanted to become a singer, and she wanted to become a doctor. (Two independent clauses – two verbs)

Complex sentence
A complex sentence also has more than one clause but of one them must be an independent clause and the other/others must be (a) dependent clause(es). There are also some particular connectors for the clauses of a complex sentence to be connected.
Example:
I know that you always wanted to be a singer. (Here, a dependent clause is followed by a connector and an independent clause. The other way around is also possible.)

Compound-complex sentence
A compound-complex sentence (or complex–compound sentence) is a mixture of the features of compound and complex sentences in one sentence. So, it must contain at least two independent clauses and at least one dependent clause.
Example:
I know that you always wanted to become a singer, but I always wanted to become a doctor. (Here, one dependent clause is followed by a complex connector and two independent clauses with a compound conjunction between them.)  

Functionally, sentences are of mainly four types:

Declarative sentence
Imperative sentence
Interrogative sentence, and
Exclamatory sentence

Declarative sentence:
An assertive sentence (declarative sentence) simply expresses an opinion/feeling, or makes a statement, or describes things. In other words, it declares something. This type of sentence ends with a period (i.e., a full-stop).
Example:
I want to be a good cricketer. (a statement)

Imperative sentence:
We use an imperative sentence to make a request or to give a command. Imperative sentences usually end with a period (i.e., a full stop), but under certain circumstances, it can end with a note of exclamation (i.e., exclamation mark).
Example:
Please sit down.

Interrogative sentence:
An interrogative sentence asks a question. Interrogative sentences must end with a note of interrogation (i.e., question mark)
Example:
When are you going to submit your assignment?

Exclamatory sentence.
An exclamatory sentence expresses the overflow of emotions. These emotions can be of happiness, wonder, sorrow, anger, etc. 
Example:
What a day it was!