The offical name for a Skillcape is "Cape of Accomplishment" and it is awarded to a player when they reach level 99 on a skill or once all quests are completed.
Note: As of right now, Skillcapes are now coded and added into the game!
Skillcapes are more "skill trails" as players are simply given a coloured train of particals that can never be disabled.
Why Skillcapes are in the gameEdit
Until Minecraft 1.7, capes are purely controlled by Mojang's server - except for the visibility. This means that capes don't even touch the MineScape server - instead it's just Mojang->you. So, the MineScape server has every ability to change whether a player has a cape, how it looks, and who can see it.
Starting in Minecraft 1.8, capes and skins are also stored on the server, and on the player's computer. However, this does not mean either can edit it. Dinnerbone has stated that both of these (skin & cape) are signed, and if the client detects the incorrect signature, it will reject it (and most likely reject a new one). Meaning that if we try to edit the file, you get no skin. Furthermore, the server can simply 'give' a cape to the player - only Mojang's servers decide that. If Mojang's server says "this player has no cape" then MineScape has no method of deciding otherwise. There are bound to be many servers that would see benifits of skin control such as this, but it's just not possible.
Now, you may be thinking "but I've seen people with OF capes!" Yes, Optifine has the ability to add custom capes - but only because it's a client modification. It has direct access to the game files and decides "Ok, _____ has a cape." We've stated multiple times that MineScape will forever be server-only, as it means less to download and it's easier to join.