Specialization
Specialization in Dungeon Crawl refers to a player's decision to invest a character's experience in only a few specific skills to train them faster and make the related abilities more powerful and reliable, rather than attempting to learn many different skills. Specializing is generally regarded as essential, since there is not enough experience in a normal game to advance more than a handful of skills to a high level. However, over-specialization can result in the character being unable to handle some of the challenges they face for lack of flexibility.
Benefits of Specialization
As mentioned above, specialization improves the power and reliability of the abilities related to the skills in which a character specializes. For example, specializing in Conjurations early on will improve your damage-dealing spells (from Magic Dart on up), making it easier and faster to blast increasingly stronger monsters. By comparison, focusing on Transmutations lets you change yourself into successively more deadly forms, as well as providing a sampling of utility and escape spells, while Summoning lets you call on various magical servants, mostly summoned to the particular fight.
Drawbacks of Specialization
Over-specialization may result in difficulty dealing with certain challenges. It routinely affects spellcasters of all walks of life: for example, a Fire Elementalist who does not branch out into other schools of magic may have a hard time defeating monsters that are resistant or even immune to fire. Similarly, a melee character who completely eschews magic may find themselves surrounded by hordes of summoned demons in the later branches without a way to thin out their numbers, such as the Abjuration spell.
How to Specialize
Specializing is easy. Press the m key to access your Skills screen and see all the skills you currently know. These will be separated by spaces into up to four categories (Offensive, Defensive, Thieving, and Magic; fewer if you don't know at least one skill in each category). Decide which skills you wish to spend experience on for now. Pressing the key associated with a skill will toggle its training On (light gray) or Off (dark gray). If you wish to make certain skills train faster than others, you can press the | key to allow skill focus, a third state for a skill in which they will receive double their fair share of the experience you gain (white). After setting your skills, simply go forth and kill things to earn experience, and it will be automatically divided between all the active skills of the categories you've recently used. Just remember that as you progress through the game, you will eventually gain entirely new skills which will be toggled On by default.
History
In Crawl versions prior to 0.9, experience was gathered and spent gradually as you performed actions related to each Skill. It was much harder to avoid unintentionally training certain things.
