basically

It frequently pops up in speech, usually at the start of a clause. The only thing it does to the semantic construct of a sentence is to make the user's statement sound slightly more certain. However, in loading a sentence with additional words, it detracts from the main message (the signal to noise ratio kicks in). Consider that of the two sentences:

Basically, you need an invitation to attend Fred's party
You need an invitation to attend Max's party

Both parties appear equally unwelcoming to those not holding invites.

Basically has a sister - 'Essentially'.

Basically, you need an invitation to attend Fred's party.

That probably means:

You don't technically need an invitation to attend Fred's party, but you'd better damn well have one if you want to have any considerable chance of getting in.

And likewise, if someone's having a conversation with me and it runs along the lines of:

Man, that John Kerry is criminally insane, isn't he?
Basically, yeah.

That last sentence means:

He's not criminally insane per se, but it's not too unreasonable to say that he is, because he's close enough.

So "basically" is a way of making a statement true in a simplified sense, but simultaneously recognizing that it is technically false in some (assumedly) insignificant way.

See? It has a simple meaning, basically.