What are the differences between Practice and KitPvP?
Practice is what the name itself is, a place for you to practice. Therefore you and your opponents will have the same kits / ladders. Ranks do not affect gameplay here. KitPvP is an open field kind of game, with many possibilities such as teaming with your friends or trying to play solo. You can get additional kits , items and effects with in-game or real-life cash. Some kits might be better than the others, while some might be more user-friendly with individuals.
What are the differences between Skills and Perks?
Skills are specialized per kits. For example, Miner kits get hastes and Archer got speed. Perks are miscellaneous items, for example capes and particles, which are not affecting the gameplay.