Ni-Cd (Ni-Cad) = Nickel Cadmium
Ni-MH = Nickel Metal Hydride
LiPo = Lithium Polymer
LiPo batteries are the same kind used in your cell phones and most modern portable electronics these days, but they require a lot of care, as they are quite volatile and can explode quite violently if mistreated in use or in charging.
If you're pulling an M120 type spring, just make sure the LiPo pack you're getting is rated for at least 20C. 25C recommended. Stick with 7.4v = 11.1v will be too hot for your mechbox.
As for the Systema springs, the standard M120 is an oil quenched steel, whereas the M120S is Stainless steel (hence, the "S"), and the M120S is more suited for a skirmishing gun that goes through full auto cycles. It seems to return springiness faster than an M120 does and is what Systema recommends for the common skirmisher. The Stainless spring also seems to push out a slightly higher velocity than the standard M120.