A religion is a set of beliefs that a group of individuals proscribe to. By that definition, Islam is certainly a religion. It has strong roots in Judaism and Christianity and believes in the prophet Mohammad and his teachings. Islam is a major religion for the middle east and has grown significantly recently.
Islam really is a religion. They have a holy book, they have followers, and they have a deity that they believe in. They also have important tenets of their religion that they believe that their followers should follow, and they also have religious holidays that are very popular and fun.
There are a whole lot of requirements to class something as either a religion or not a religion. For example, believing in the Flying Spaghetti Monster is, actually, a legitimate religious belief. It has certain scriptures and a following. That is why Islam, which definitely has core writings, a belief in a god, and a large following, can be considered a religion.
More like a cult. The definition of religion is, "the belief in and worship of a superhuman controlling power, especially a personal God or gods". SO yes Islam may have this trait, however, their actions just portray a callous behaviour. Yes many religions have gone through their time where they may have done extreme things, however they've learnt. For Islam, their time has just began and so god forbid us with all their actions, in the name of "religion". A cult however is, "a relatively small group of people having religious beliefs or practices regarded by others as strange or sinister." Which to me, portrays more of what islam is. However, it does not apply to all muslims.