Hills are traversable by infantry without problems, but armor gets stuck in hills all the time, not to mention how hard it is to aim. Obviously, armor is of little use in the hills and so is anti-armor. Depending on the elevation differences, assult infantry is usually the only squad worth sending into this hellish place. Some rifles here and there can help with slowing down the enemy, but your main focus will still be close combat infantry. Mortars are very effective in the hills, but they have problems with ammunition, unless a supply depot is made somewhere near.
Tactics-wise there is nothing out of the ordinary - you want to take high points in the hills to allow your longer ranged support infntry to deal some damage before you are forced to charge down. Battles in hills are always very quick due to the high damage units used, but winning wars is almost impossible - hills rarely offer an economic bonus and bases are never placed in the hilles either, so you'll be forced to exit them at some point with nothing but short-ranged infantry and maybe some support troops, while rifles and tanks get the advantage right away.
Hills do offer many options for extra infantry pushes and are often used for sneak attacks to disorient the enemy and force him to move away from the main force.
For examples of hills, check the map "road to rome", most notably the lower left corner of the map, as it has a rather large hill section with many useful exit points should you take over the hills.