And is a common conjunction in the English language.

It is used to signify truth in both clauses it is connected to. Unlike some other conjunctions however, such as but or because, its conjunctions must always be in the same order:
    clause conjunction clause
    "This is an example and it is correct."

Other conjunctions may be connected in this way but and may not be, in practice:
    conjunction clause, clause
    "And this is an example, it is technically correct, but in practice it appears incorrect."
When heard, a previous sentence is expected to be connected. The cause of this is possibly abundant use of the following syntax.

A common, but incorrect, way of using and is this:
    clause. conjunction clause
    "This is an example. And it is incorrect."
This, however, is technically incorrect. The first clause has ended with a period, then it is referenced from the conjunction.

Oolong says "But there's some debate as to whether it is *really* incorrect to start a sentence with 'and' (in the 'clause. Conjunction, clause' form), I believe. There are times when it makes perfect sense and is quite hard to avoid, after all."