Grab the Searing Bond, link it with Elemental Focus, Increased Burning and Rapid Decay. Then you just drop two Flame Totems and one Searing Bond, and the Searing Bond will throw a beam from itself to you and both of the Flame Totems. Now, the beams don't (as far as I've been told) stack, so it's enough to have one beam touching a monster. The Flame Totems will never replace a Searing Bond and vice versa, so it works like a charm.
On top of that, have Flammability and throw that on the tougher bosses.
Maps are just extra zones you can run, that have a bunch of monsters and a boss at the end. You get the lowest level maps as drops on Merciless difficulty, and then inside those maps you can get higher level maps, and so on. You open the maps with a map device, and you get a limited number of portals you can use to go back if you come out to sell stuff or die.
That gives you access to the new Ascendancy class, or "subclass" for your Witch.
http://pathofexile.gamepedia.com/Ascendancy_class
Personally I haven't checked out the subclasses yet from the wiki, but the Chieftain on Marauder has been designed for fire totems, so it's pretty cool.
Yeah, you can do maps no problem, as well as get your subclass and it's Ascendancy points.
On the topic of getting your subclass, the Labyrinth (where you get it from) was just nerfed;
https://www.pathofexile.com/forum/view-thread/1612488