Combat Level, also known as the player's level, or CL, is calculated by:
If (Acc+Str+Def+Health+Arch) / 4 > Magic, then magic divider = 6, else divider = 4
If (Acc+Str+Def+Health+Magic) / 4 > Arch, then archery divider = 6, else divider = 4
Combat Level = (Acc+Str+Def+Health)/4 + (Magic / magic divider) + (Arch / archery divider)
To view your CL either type the command in chat /combats and look for your name and level amongst the other players on the world you are on, or if you have /mods enabled, type the command in chat /cl or mouse over your character for pop-up info.
Combat Level is used for creating and joining Party Quest Dungeons (PQs), or for PvP in NML to determine who can attack you, and to determine which aggressive monsters will initiate an attack. Aggressive monsters won't initiate an attack if they are 20 or more levels below the player's combat level (except in Party Quest Dungeons and NML).
It is also a fairly bad way to determine one's overall fighting capability, as Combat Level is represented by one's total base stats, since characters that specialize in one or two combat skills tend to be more powerful than those with fairly equal levels in all six at the same CL. This is because equipment stat values rise exponentially with increase of level required to wear them, thus better equipment can be worn at lower levels.
A better way to determine one's overall fighting capability is by directly comparing total adjusted stats with those of a mob; this can be accomplished via mouse over, or on touchscreens you may poke the square the mob is on through a menu (such as inventory), or with /mods enabled, you can Right Click a Mob either in game or in the /wiki and select Combat Analysis for average/max damage and chance to hit for both you and the mob, along with an estimated total if you attempt to fight the mob to death without running away.
Here is a link to an off site web page you can use to enter your combat skills and calculate your level. Use it to change values to see what would happen to your Combat Level if you raised Archery, for example.