Difference between a hammer drill and other drills?

So I'm looking at Dewalt drills, Dewalt just because my dad has a dewalt and I love the thing compared to other drills ive used. So, Dewalt has a 18V Lithium Ion battery powered "Hammer Drill" and a 18V Lithium Ion battery powered "Drill / Driver" ... show more So I'm looking at Dewalt drills, Dewalt just because my dad has a dewalt and I love the thing compared to other drills ive used. So, Dewalt has a 18V Lithium Ion battery powered "Hammer Drill" and a 18V Lithium Ion battery powered "Drill / Driver"

http://www.sears.com/shc/s/Compare?storeId=10153&catalogId=12605&undefined&partNumber_1=00928368000P&partNumber_2=00917350000P&prodCount=2

What's the difference?

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Update: @adaviel

So is a hammer drill kind of like a drill / impact driver combo?
3 answers 3