How to get a modded minecraft server?

I can get the normal minecraft, without any mods, to run a server properly. However, when I try to run a server that is modded, it keeps crashing.
I don't know why it is doing that. Anyone can explain please?
Trying to mod a 1.8 version, of pixelmon on a server.

Brandon 28.09.2015

Pixelmon is an EXTREMELY huge mod, and even on a regular singleplayer game, can run slow. Now add 1000s of people and pokémon running around and you have got yourself a ton of lag. It is most likely your PC or laptop. The gamers you see playing on those servers on YouTube have expensive computers and good internet. If you upgrade those, it should run.

Odyssey 28.09.2015