Can someone explain what neutrinoes are?

[H]bunbunbunbun

I know they have no charge, vitually no mass, a by-product in decays(to conserve lepton number),

but is there any other properties/ do they do anything??[[tk_42]]
(like, if electron neutrino and anti eletron neutrino collide, will they annihilate?)
my physics teacher crammed us with loads of stuff in two double lessons[[tk_35]]
so much confusion