Basic ammunition types:
ECP: Electron Charge Pack. Used by the Laser RCW and Gatling Laser, as well as the Tesla Cannon.
SEC: Small Energy Cell. Used by the laser pistol, plasma pistol, and plasma defender.
MFC: Micro-Fusion Cell. Used by the laser rifle, plasma rifle, tri-beam laser, multiplas rifle, gauss rifle, and plasma caster
2 ECPs make 1 SEC, or 3 ECPs make 1 MFC. Think of them as increasingly large types of batteries.
Once your science skill is high enough, you can also make:
Overcharged ammo: Does more damage than usual and has improved armor penetration, but increases weapon wear and is less efficient.
Maxcharged ammo: Does even more damage, with even better armor penetration, but again increases weapon wear and is less efficient.
You can also Recycle "drained" cells if you have Science skill. Drained cells are created after you shoot energy weapons that use cells, or sometimes found on enemies that use them.