Explosive Rounds (And EX-Explosive Rounds) are mini-shaped charges, essentially. They don't fragment on impact, they detonate with a very small cratering effect. (Think less "Grenade" and more "Anti-Tank". Only a LOOOOOOOOOOOOOT smaller!).
Explosive rounds are a cheaper, generic brand, and they use less powerful explosives. EX-Explosive is a brand name with more "Pop" for your nuyen. They will work with silencers, which will muffle the muzzle blast, but not the whip-crack of supersonic rounds or the "POP" of the round hitting it's target. So it's quieter, but not exactly QUIET, if you catch my meaning.