Most 
SKILLS allow you to boost particular stats in the game. Some give the ability to perform certain actions (like Lockpicking). All characters start with one skill. Mages and Druids gain one additional skill every level. All other classes gain one skill at each odd level (e.g., 3, 5, 7, …) and two skills at each even level (e.g., 2, 4, 6, …).
Skills are classified into four categories: 
OFFENSE, 
DEFENSE, 
SUPPORT and 
ARMOR. Despite this distinction, they’re mostly used just for logical grouping and when characters level up they can choose from any of these categories (the exception to this is if you do skills training in town — those are specific to a given category). Skills also have minimum level requirements, so as you level up you will see more skills become available.
At level 1, only particular OFFENSE skills are available (the ones related to specific weapon types). This is to simplify the process of selecting the most beneficial skill for newbies. The list of skills can be quite large and figuring out which to choose can be daunting. A large group of skills becomes available at level 2.
Some skills can be negative, although these are specific to given classes or races and none of these are available to choose from when leveling up. Most classes have both a positive and a negative skill that is automatically assigned. For example, Fighters automatically get Tough as Nails (Fighter) which gives them 25% 
WEAK RESISTANCE. But they also get Fear of Magik (Fighter), which lowers 
MAGIK DEFENSE by 10%. Those two are exclusive to Fighters and automatic — they do not require any skill points to be expended. There is another skill named Tough as Nails that anyone (including Fighters) can pick up and will also give 25% WEAK RESISTANCE (skills stack, so a Fighter that does that would then have 50% WEAK RESISTANCE in all).
Some skills allow you to acquire them more than once in order to multiply the benefits. Generally, you’ll see something like “(x1/2)” listed near the skill. In this case, this would indicate that you have 1 of the maximum 2 instances of this skill, and you could add it again.
Many skills have a base version plus an Advanced, Superior and Expert version starting at levels 10, 20 and 30 respectively. The higher category skills require the lower-level associated skill to be acquired before you can see it in the list to choose from. For example, there is the One-Handed Swords skill which gives +5 ACC and +10% DMG when using a one-handed sword and can be acquired twice. Upon reaching level 10, if you’ve acquired that skill at least once you will see the Advanced One-Handed Swords skill, which also gives +5 ACC and +10% DMG and can be acquired twice. If you get that one at least once, at level 20 you will see the Superior One-Handed Swords skill. And Expert One-Handed Swords at level 30.
WEAPON skills that grant 
ACC and Damage (
DMG) bonuses for particular weapon types are especially useful. Generally, there are two of these for every weapon type every 10 levels as previously mentioned, and these are some of the most advantageous skills to acquire. In fact, you must pick one of these skills at level 1 and it is recommended that you get the other one for the same weapon type at level 2. The section in the guide on Weapons will cover more details, but it is worth noting at this point that most characters should stick to one weapon type to invest skills in and not try to spread them between multiple weapon types (e.g., don’t try to specialize in both one-handed swords and one-handed hammers — choose one and stick to it). Having said that, it may be advantageous to specialize in both a melee and a ranged weapon, depending on your playing style and preferences.
Many OFFENSE skills also will boost particular status effects or elemental damage types. An example of this is Hot-Headed, which can boost the accuracy of 
fire-based attacks by 5% and damage by 10%, or Headbanger which can increase the chance of 
STUN effects by 5% and their duration by 1 round. Note that these types of skills boost existing weapon features or character abilities. They do not grant them. If a character has an ability that does fire damage then Hot-Headed will boost that. But just attacking with a plain sword will not do fire damage from having this skill. But if you have Headbanger and attack with a normal hammer (which has an inherent chance to stun) its effects will be amplified.
There are several OFFENSE skills that grant accuracy and damage bonuses against certain monster types, like Goblinologist or Herpetologist. These generally give a 15% ACC and 
DEF plus 10% DMG bonus against monsters in that category. A character may only possess one of these monster-type skills, so if you pick Goblinologist you may not pick Herpetologist or any other one.
Some skills will boost the range, accuracy or damage of categories of weapons or abilities. For example, there are skills that will boost the distance for ranged weapons by 1 (that will affect bows, crossbows, wands, slingshots, or staves). Other skills can boost the range for 
HOLY, 
FLASK, 
PSYCHIC or 
MAGIK abilities (these are class-specific).
Many DEFENSE skills will either boost your DEF or MGK DEF, or give you increased RESISTANCE to certain effects. If you prefer using shields, a good one to have is Shield Handling or Shield Defense. These skills will (respectively) lower the shield 
ACC PENALTY by 25% or raise the 
BLOCKING by 10% for each version (Base, Advanced, Superior, Expert) for which you acquire it.
The SUPPORT category has many useful skills, some of which can boost movement, initiative, health, stamina, stamina regeneration, night-vision, healing powers, carrying weight, plus more. You are more than likely to invest in Fleet-Footed, which grants +1 MP, or In it for the Long Haul, which grants +10% stamina. Don’t neglect SUPPORT skills as these are some of the best of the bunch!
ARMOR skills will increase the damage mitigation amount that different armor types will do, as well as lower their weight and reduce the degradation rate from taking damage, so these are useful. Even if you tend to “mix-and-match” different types of armor (e.g., plate chestpiece but leather gloves) it can be beneficial to get one or more covering the types that you typically wear.