Jammer only conceals you. Yes, the category is a bit odd, but I suppose MF wanted less pages of weapons with more content on each and couldn't find a better classification.
Correct on the second part. You will not be able to see your enemy on radar nor can they see you. Jammers block out the equipped player on both radar mk1 and radar mk2 for all enemy radars. (Still visible to teammates if anybody is wondering.)
Keep in mind the roommate's radar is also limited to a range around them. I sometimes do recon at or behind enemy lines with radar and jammer, using silenced Maurauder and commanding teammates when to move up with quick command icons.
That's the other annoying thing.When you do quick command icons, your enemy can see that massive bubble above your head (not sure if they can hear your command too though). So you have to time it right if you're commanding behind enemy lines.
I do however, know that you can hear your enemies commands after your dead.