It is a a very good question and a complicated one too in this modern world.
There are many definitions of religion based on faith, belief system, observances etc.
Let’s take a step back and look into the past how it might have come into existence. I’ll try to explain with a very simple basic approach.
Assume that there are several isolated areas different in the terms of availability of basic human needs such as food and shelter. On this each separate land humans evolved in their own way. They were babies, then grew into adults and established a small colony. These colonies decided not to follow the rules of nature for survival and invented the tools and the techniques to overpower the beasts and stay on top of the food chain. Going forward they devised certain rules-of-engagement like not to fight among each other, share food equally among each other and live together in harmony.
Slowly, the community started growing, developing into colonies, and so thus the pressure on survival and the individuality. More rules came into existence. To ease these pressures a leader was chosen who will be responsible to maintain peace and harmony in the community. This leader, with the help of the elder members, devised some basic norms to maintain peace and harmony in the society. To enforce these norms the concept of “virtue” and “evil” came into existence. This gave birth to a sleuth of value systems. These values were tied to “guilt” and “pleasure” and were enforced onto these colonies. A governance was inducted with the idea of the “moral duties” and the “punishment” system. To further strengthen these value systems a concept of symbolic places like “Heaven” and “Hell” were theorised and the ruler of these symbolic places was designated to “Gods” and “Demons“.
Later as time progressed some of these colonies started migrating due to climate change drying rivers and affecting rainfall. These migrated colonies started to settle beside the old established colonies. They were different to each other in terms of their rules, norms and value systems. Each colony wanted to maintain their own rituals. This gave birth to the so-called religion to simply identify themselves from one another. Its purpose was just to identify one community from another. Its primary goal was the same. Later, for better coherence and commonality, these settlements gave rise to the kingdoms and thus the political boundaries were created.
In a nutshell Religion is a body of rules, norms and value systems governing the conduct of human affairs to create a “Humane” society.
To know more facts about religion – Visit http://www.religionfacts.com/
Watch this video which shows the timeline of the different religions spread around the world.