A clone is two chartacters that have same movesets. Some are a bit a rip-off the characters.
Type of Clones
Mario & Luigi
Mario is the original one and Luigi is the clone. Both have the same standard, down aerial and up attack. The difference between them is that Luigi is the one that needs to be unlockable and their side attack is different. Mario is the Punch-Kick Combo, which Mario punches then kicks in the direction he is facing and Luigi is the Green Missile, which he would launch himself and hit with his head.
Sonic & Super Sonic
Both of them have the same moveset, just the Super Sonic is faster than Sonic, making Sonic the second fastest in the game.
Link & Young Link
Both of them have the same attacks, just that Young Link is shorter, making it more easier to dodge attacks.
Blade & Blue
Both of them have the same attacks, just that their appearance is difference.