Can I download torrents on Mac without any problems?

At this trime there are no viruses that can infect a Mac OS X computer. There are, or were, a few Trojans, but they have been taken care of, AFAIK.

So the answer to your question is YES you can download torrents to your Mac.

Whether a torrent is safe depends greatly on what you're downloading and from whom. That's not often easy to identify with torrents, which are designed to allow you to download from complete and total strangers.

I would advise not to use torrents at all. Most often, torrents are simply a source for illegal material, and you really don't want to trust people who are stealing music, movies and software. Would you take a pill given to you by a complete stranger in a bad part of town? No. Neither should you download something illegal from a complete stranger, as it may be dangerous.

If you insist on using torrents for legitimate purposes, you need to protect yourself. First, only use a torrent that supports checksums of downloaded content, which can be compared to a checksum of the original file to verify that it hasn't been tampered with. (As I don't use torrents, I can't recommend which ones do that.) You will also need to run good anti-virus software, such as Sophos.

If you decide to avoid torrents, you can easily get by without anti-virus software. See my Mac Malware Guide.

If you download and use pirated commercial software, you can assume that you'll be infected with malware. The same goes for software that isn't necessarily pirated, such as "codecs," "players," or "archive extractors." No "anti-virus" software will protect you. At most, it will only delay the infection for a while.

With BitTorrent clients, you're bypassing all the built-in protection against malware.

If you download only data files, then the most you have to worry about is a federal lawsuit that will reduce to you to penury, even if you win.

What's can happen if i download an infected torrent..

Anything. Your computer will become infected. Might steal passwords, banking info, change DNS to redirect, among any number of possibilities. A virus scanner won't necessarily pick up any malware present, nor will 10.8's built in XProtect. If you are DLing pirated stuff especially, you will have a good chance of getting malware.

All depends on the torrent and whatever it is you are downloading.

There are, or were, a few Trojans, but they have been taken care of, AFAIK.

Depends on what you mean by "taken care of." There are still a number of them floating around, though they're all pretty uncommon at this point. They're all taken care of by protection built into Mac OS X, but most torrent apps actually bypass those protections. Plus, this can change easily, and probably will again at some point. We had a major malware outbreak in 2011 (MacDefender) and another in 2012 (Flashback), and the gang behind both of those is still on the loose. I wouldn't be surprised if they have some grand plans for 2013.

Downloading illegal material from torrents, or even legal material from the wrong torrents, is still highly risky behavior.