Nothing really comes to mind to compare it with, it was and still is a masterpiece when it comes to RTS and nobody really got close to it. Basicly you start with a mothership that act as a main "base" where you can do research and build, the key is to find a balance between your attack ships (fighters, frigates and capital ships) to defeat ze enemy. You can move in any direction (space, duh) and its all quite tactical yet easy to understand.
If you like playing either RTS games or just space games this is a must play for sure.