Like PCs, operating systems in smartphones in some cases become irrelevant. A lot of OS upgrades are provided over-the-air by wireless carriers, or users can manully upgrade the OS through distributions -- or ROMs -- that can be found on sites like Cyanogen or XDA-Developers. 

These independent distros come with many advanced features but also carry a fair share of risks as they haven't been verified by wireless providers. Some features like Wi-Fi in the smartphone may not work, just to give you an idea. Also keep in mind that loading these independent ROMs is very very risky, and only for experts who know what they are doing. An upgrade gone wrong could "brick" the smartphone, leaving it unusable for the rest of its life.

A number of new smartphones are based on Android, a Linux-based OS.  You have to match a ROM with the smartphone you have -- you can't just load any ROM on any smartphone. And there's a process to follow -- it starts with rooting your smartphone with the help of programs like Superoneclick or Universal Androot. That enables the process which ends with the new ROM to be flashed. But closely follow the procedures provided.

Modding is dangerous, and only for experts who know what their doing, or experimenting. *Do not mod or try to flash a new ROM on your smartphone unless you know what you are doing.* If you are happy with the way things work on your smartphone, there's no point loading a new ROM. Just wait to buy a new device or wait for an OS update pushed by your carrier.