And is a common [conjunction] in the [English] language.

It is used to signify truth in both [clause|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."

and (thing)