Wine is an open source implementation of Windows APIs for non-Windows platforms... allowing Windows programs to run without Windows. It started for Linux but has support for many OSes now, including Mac OS X. Normal Wine for OSX is build it yourself and all command line, so its best to use another tool for OSX that uses Wine, so you don't have to do it all yourself.
a fork off Wine is what ended up being called Cider that Transgaming uses for official Mac games sold that say they use "Cider"
Crossover is probably the most well known product for Linux and Mac OS X that uses Wine as well.
Wineskin is my own project... made as a tool to make a port of Windows software over to OSX and have it run like a normal native .app, much like Transgaming does with Cider, but all open source so anyone can use it, not just game companies that have tons of money.
Wine =
http://www.winehq.org/
Crossover =
http://www.codeweavers.com/
Wineskin =
http://wineskin.doh123.com/
Being that this is all done by people with no help from MS in any way... its not 100% done, or working 100% right... its a work in progress. So while many things work, and work well, there are some software packages that won't work right, and many that won't work at all... so it takes some research to get it all down right for the title your wanting to run... and being that DirectX games have to be translated to use OpenGL, most highly graphical games will run much slower than natively on Windows, but still faster than a Virtual Machine.
Wineskin I made for people to make their own ports and wrappers... people who make the wrappers can share them with others, or companies can use Wineskin to do their own ports. A few companies have made mac versions of their product with Wineskin... no really big names... and the Fallen Earth MMORPG is using Wineskin for its Mac beta. Its never as good as a native port can be, but is sometimes better than native ports turn out.... and is much cheaper... which is why game companies use Cider so much.
You can often have someone else do the work for you to get a game working, and then you just play it... check out
www.portingteam.com .. its a Mac gaming community where a lot of porters make wrappers and such, so you can just download the one made for the game you want to play and install your game and play, without having to know how it works or all the tedious things to try in troubleshooting to get it to work.