When applied to a character, the term "level" can refer to a number of things.
Security Level / Threat Level
A character's Security Level (for Heroes) or Threat Level (for Villains) reflects the number of times the character has visited a Trainer to level up...plus one, since newly-created characters are Security/Threat Level 1 without ever seeing a trainer.
A character's Experience Level is the maximum Security/Threat Level that the character has the experience to reach, regardless of whether he has actually trained enough times to reach it.
This level is compared against Enhancement levels to determine their relative potency. It is also the default value for a character's Combat Level (see below).
Combat Level / Effective Level
A character's Combat Level, also called his Effective Level, is normally equal to his Experience Level. The exception is characters who have been Sidekicked or Exemplared. If he is Sidekicked to another character, his Combat Level will be one below that character's. If he is Exemplared to another, his Combat Level will be equal to the other's. If he is automatically Sidekicked or Exemplared due to being in a Task Force or a PvP Zone, his Combat Level will be a predetermined number specific to that TF or zone.
A character's Combat Level determines his base health and the base strength of all his powers. It, along with the Combat Level of the character's enemies, affects his chances of hitting and getting hit by them.
Another term for notoriety, the setting that controls the difficulty of missions and task forces relative to your security/threat level.