全然(ぜんぜん)(Not) at all

In Japanese, there are several adverbs that are traditionally used exclusively with negative expressions (containing ない). 全然(ぜんぜん) is one of them. When used at the beginning of a ない (or ません) containing phrase, 全然（ぜんぜん) has the meaning of 'completely not (A)', or 'not (A) at all'.

In writing, it is equally as natural to use either 全然(ぜんぜん) as kanji, or hiragana only, so both forms may be used confidently.

ぜんぜん is more flexible (and friendly sounding) in casual conversation than 全(まった)く, a similar expression which has a formal (often negative) nuance.

While both of these sentences have the meaning of 'not (A) at all', まったく could give the listener the impression that you are not pleased with the situation. This will depend quite a bit on tone of voice, though.

Caution

Despite originally being used only with negative expressions, it is becoming more and more common for ぜんぜん to appear with positive expressions as well. In these cases, it just has the meaning of 'totally (A)', or 'completely (A)'.

Although this is technically not grammatically correct, it is becoming so frequently used that it is considered correct by most people (especially the younger generation).