This is basically how I order my skills for my chars in Elsword.
If you don't understand it, here's some stuff to help.
Unlike most MMOs, Elsword uses preset characters to start you off. Each character has three paths you can choose from (or in Rose's case, four), each one having different commands and skills, and each character has their own unique system. Additionally, Elsword plays like a beat-em-up, unlike most of the MMOs I've played.
Flexibility skills are Active-type skills. They're weaker than Special Actives, and unlike Special Actives, they don't give invincibility frames.
Buff skills give a buff to the user and all non-enemy players in range (this game has a PvP mode). The only exceptions are Rose and Ain, as their buff skills only grant buffs to the caster (only one of Ain's paths has a buff skill).
Tenacity skills are on the weak side of Special Actives.
Strength skills are around the middle of Special Actives.
Bravery skills are on the strong side of Special Actives.
Hyper skills are Hyper Actives, which are stronger than Special Actives, but always cost 300MP, and consume an El's Essence when used. El's Essences are items that are obtainable in-game (the quest to unlock Hyper Active skills teaches you how to obtain them)
The red and blue colorations are because of the character systems of two characters: Elsword and Elesis. Their skills and commands are divided into red and blue, both of which affect their character system. However, the reason the 4-slot and 5-slot parts do not have colors is because there's no real way to balance the colors without at least 8 slots.
There is a dual character that is playable in the game called LuCiel, being a pair made of a young-looking demon girl (Lu) and a half-demon man (Ciel). Their Tenacity and Strength skills are "Switch" skills, which switches to one of the characters when the other is in play (Lu skills switch to Lu when using Ciel and vise-versa (There are some Active "switch" skills that will switch you to the other character regardless of which one you're using at the time, but I never use them)). When switching this way, they gain points for their Combination Meter, which can hold a maximum of 10 points at any time. Alternatively, certain Commands will switch between the two, but they only give Combination Points when the attack makes contact with an enemy. When the meter is full, it is used for their Bravery skills, known as "Combination" skills, which do not consume MP, but require and consume 10 Combination Points when used. Due to how Switch Skills work, I only use LuCiel Strength Skills when I have at least 8 Skill Slots.
I always have exactly one skill that is shared between all copies of a character. The reason that it is not colored is because I want to keep it at four of each color, and each of them only have one Flexibility skill at base class that is neither color, as they do not do any damage without Passive effects, instead granting the users Super Armor. Elsword's skill is called "Iron Body", and Elesis' skill is "Iron Body - Strong".
And finally, the number of slots themselves. Every character starts out with 4 slots, but 4 additional slots can be bought from the Item Mall right away. These 4 slots are also temporarily unlocked when achieving first or second job advancement. Additionally, when a character achieves "Transcendence", which is basically the highest advancement of every path, one more slot is permanently unlocked for free, but the 10th and final slot is only temporarily unlocked, which is again sold in the Item Mall (this slot can only be unlocked after Transcending). Fortunately, the items that permanently unlock the top 5 slots are tradable, so you can find them on the market, which is accessed from the board in any town/rest area.