Aptitude is a measure of how fast you can advance in a skill. You have a separate aptitude rating for each skill, and the higher your aptitude, the less experience you will need to advance the skill in question. Aptitudes are rated from -5 to 5, with 0 being average; the higher over that you are for your main skills, the better.
Your aptitudes are determined solely by your species.
As a result, certain species are biased or specialized towards certain backgrounds. For example, Deep Elves perform much better at magical backgrounds than purely melee-oriented ones, while the reverse is true for Minotaurs. One of the keys to winning the game is paying attention to your species' aptitudes, and choosing a playstyle that takes the most advantage of them. Luckily, the character select screen helps you with that, as backgrounds that your species has good aptitudes for are highlighted as "recommended".
?% in-game to see the big table of races and aptitudes.
Some skills are naturally harder or easier to level for all species. Spellcasting takes 130% the normal amount of exp needed to level up, while invocations, evocations and stealth require only 80% of the normal amount of exp to level up.
See the Talk:Aptitude page for some detailed numbers.
Aptitude / XP formula
If you're interested in the actual math, an aptitude of
n means that you need
2^(-n/4) times as much exp to advance as a human.
Example: A Spriggan has a +4 aptitude in Stabbing. If it would take a Human 100 experience points to advance a level in Stabbing, it will take the Spriggan
2^(-4/4) -> 2^(-1) -> 0.5 times as much experience, or 50 experience points to advance a level.
The table below shows the relative experience needed to go up a skill level, assuming that with the aptitude of 0, 100 experience is needed. For example an aptitude of -4 means 200 experience is required to go up a skill level, or twice as much as aptitude 0.
Note that for versions of Crawl older than version 0.7.0, aptitudes were expressed in numbers smaller or larger than 100 (average), as in the Skill Points table below.
|-4||(200)||very poor aptitude (learning half as fast as Humans)|
|-1||(119)||slightly disfavoured aptitude|
|0||(100)||standard aptitude (Humans)|
|+1||(84)||slightly favoured aptitude|
|+3||(59)||very strong aptitude|
|+4||(50)||outstanding aptitude (learning twice as fast as Humans)|