There's an old belief that Macs don't have the same crapware/malware problem that Windows does. However, with the rise of Macs over the past few years, that's no longer true. Our friends at the How-To Geek explain how OS X crapware works, and how to avoid it.
Much like on Windows, Mac adware typically comes from fake links in search engines and "bundle download" sites like Download.com. One of the worst offenders? Fake versions of VLC. Make a quick search on Bing or Yahoo (Google blocks crapware results) for VLC and you'll find a ton of fake installers (like the one pictured above) that put adware on your system. If you do end up with any of this adware, it tends to hijack your browser's new tab search and home pages to bring up obnoxious support warnings trying to trick you into installing even more adware.
Most of these stick to your browser as obnoxious old adware, but How-To Geek found that some can dig even deeper:
These more serious pieces of malware install themselves as a daemon, or service, that runs in the background and behind the scenes. You can find these things in the /Library/LaunchAgents or /Library/LaunchDaemons folder, which will have some really weird looking items that just don't belong. This folder could also be used for real things from real applications, so don't go cleaning out this folder entirely or anything.
Of course, like Windows, you'll have to jump through some hoops to actually install this stuff, and any educated Mac user will recognise most of this software as fishy. However, since most people don't think Macs have this problem, they might let their guard down.
To keep this stuff off your system (and off your family and friend's computers), make sure you can only download approved apps from the Mac App Store. Head to System Preferences > Security & Privacy and make sure the box under "Allow apps downloaded from" is set to Mac App Store and identified developers. Of course, when you absolutely have to download something from an unapproved developer, be sure to read the installer carefully and avoid agreeing to any other installations.
If you already have adware on your computer, we like Adware Medic as a simple, on-demand anti-adware scanner that will help you get rid of anything already on your system. How-To Geek goes deep into how all this adware and malware works, so check out their full post for more info.