Designate MA - the PC you (as leader) choose with this ability will have a pretty white ring around his target at all times. Throw it on the tank or the puller and have everyone beat on the white ringed target. This is pretty much set it and forget it, and invaluable when you're having crowd control issues - when people actually pay attention, you end up with a lot less damage in the form of broken mezzes or confusion about exactly what mob is rooted and what mob should be moved. Heck, it even helps when you have no crowd control at all, as having everyone beating on the same mob = 1/2 incoming damage that much sooner when you get an add, as opposed to everyone but ranger #1 beating on the right mob. As a secondary convenience, once this ability is purchased, anyone in your group can use the command "/assist group" to get the MA's target, and thus do not have to change their assist button every time you change pullers.
Mark NPC - You get the ability to hit a hotbutton to put a ring around the mob you're targetting. With multiple levels, you can mark multiple npcs at once. This is a more active ability, as the ring stays with the NPC even if you retarget. Delegate Mark NPC allows you to give this same ability to one of your party members.
You can have both of these abilities going at once, as the rings look different. Personally, I find Designate Main Assist to be invaluable, whereas Mark NPC is a convenience. I've gotten DMA as my first leadership ability on all the characters I have working on leadership, and I've never regretted it.