Well it's evil, fine.
But what's evil in it?
Why not use the tag?
all browser support it?
No, although support is annoyingly broad.
can/ cannot use what controls in the taag
Can you rephrase this. I think I can decrypt the meaning of 'taag', but the words don't form a sentence.
why not to use the tag?
First and foremost, on a purely practical front — it makes text scroll.
Scrolling text is an excellent way to display a large amount of information in a small place, non-interactively. This makes it rather good for things like the "What stations this train will stop at" display that I get to look at on my commute.
A web page, however, is typically viewed in a relatively large display by a single person, who has a mouse, trackpad or other way to indicate when they have finished reading whatever is displayed. This means they do not have to wait for the start of the text, the text will not move faster then they can read properly, and the text will not move more slowly then they are comfortable with. Additionally, if they are trying to read something else that is near the scrolling text, they won't be constantly distracted by the movement in the corner of their eye.
Aside from that. It isn't yet standardized, and the closest it appears it will ever come is to be marked as obsolete by HTML 5.