Oh, so you don't really know much about competitive battling? Check out
this link to learn more about it. Basically, Pokémon are separated into
tiers depending on their usage and power. OverUsed (OU) is the standard tier of the metagame which most people play; UnderUsed (UU) is composed of Pokémon which are not powerful enough to compete in OU; NeverUsed (NU) consists of extremely weak Pokémon; and Ubers is the tier which includes legendaries like Kyogre and Dialga.
Charizard is NU because Stealth Rock, which is very common, removes half of its health upon switching in. This makes it easier to be killed. I can't really remember your team, but I remember seeing an Electrode, which is also NU (mainly because it is very frail). Generally speaking, it is recommended to consist your team of only OU Pokémon (initially) so that you focus on learning the basics.