The X-men is the Marvel Comics series about a diverse team of mutants who gained super powers through evolution with a wide range of powers meant to appeal to many fans, but our theory on which characters people give a shit about is represented here:
Mutants are a evovled form of humanity that have special powers ranging from useful things such as controlling the weather or healing from any injury, to completely useless, like Jubilee's entire existance. Most of them experience a world filled with the same level of bigotry as 1950s Alabama, so many mutants feel the need to join up with teams, Such as The X-men, or their rivals such as The Brotherhood Of Mutants.
The X-men began when a telepathic and flithy rich mutant named Charles Xaiver decided he was bored just sitting around in his wheelchair all day and got a hobby. However, said hobby involved creating a school for young mutants to control their powers, and later join them in his own private army to help fight evil mutants, aliens, and robots in order to protect humanity and raise awareness to their cause. Unforunately for them, humanity wants about as much to do with them as most NBA players do with their illegitimate children.
The Original X-men Team was just 5 members, but since none of them happened to be Wolverine nobody really cared about them until he showed up in the 1970s. Since then, membership into the X-men has grown into the hundreds, including multiple teams, providing with more security around the world and allowing Marvel to beat the dead horse until nothing but a fine paste is left.