All religions basically prescribe a way of living that is commonly accepted by people to be a good thing. By enforcing their tenets, they improve the quality of life of followers and discourage unsavory activities. The tenets are handily disclosed in exhaustive scripture. The rewards and punishments usually have to do with the afterlife. Christians have hell to fear, Buddhists must live the right way in order to break the cycle of reincarnation, Vikings are denied a place in Valhalla if they die poorly, etc etc. Priests are usually on hand to convey the word of god and dispense his blessings or denounce the unholy. Holy men are responsible for the psychological health of their religion's followers. They handle stuff like marriages, absolution, exorcism, housewarming ceremonies and other rituals that assure people that randomness is in their favor.

Religions also try to address a lot of the unknowns about the world. Some take a prescriptive approach and tell you all the details up front, like the Bible. Others take the big picture and look at why things are the way they are. Hinduism is an example of this. It's filled with allegory and ultimately boils down to the idea that everything is one, and all things are manifestation of the same energy. The fantastic elements of religion like the magical stuff and the monsters and whatnot are there to awe little kids and because dragons are awesome.

All in all, religions are actually quite similar.

The majority of differences in religions arise from the fact that different lifestyles and societal structures were perceived as the norm in different places and times. So when two religions clash they seem wacky to each other because of wildly different circumstances. Another thing is that no one's writing any new scriptures. Any changes to religions have come through major upheavals and revolutions. Modern society suffers from problems like creationism and islamic extremism because no one's shown up with a new set of tablets to update the laws and let people know being gay is okay.

TLDR; Most religions are basically an additional means to get people to conform to a standardized way of living. They do it differently, but they're doing the same thing.