While it is widely recognized that anyone can learn any skill, if they are willing to put their minds to it, it is also widely recognized that some people are simply more apt at learning certain skills than others. Some people are natural musicians and pick up the concepts of music theory easily, but have to struggle hard to learn how to operate computers; some take to fighting as a duck takes to water, but have no comprehension of the mental disciplines that make magic work. Aptitudes are a method of achieving both of these considerations.
An aptitude does not determine what you know, or even decide what you can learn. All it does is determine how difficult it will be for you to learn different types of skills. In game terms, it does this by making some skills cost more and others cost less. (These costs are measured in development points, not character points. You will use character points to buy the aptitudes now, and development points later to buy the skills.) By making certain skills more cost-effective, aptitudes influence characters to follow the template set during their creation, without forcing or limiting them to it.
All the skills a character may learn fall into ten categories (but note some skills will fall into more than one category; more on this later).
The character will have a number ranging from 1 to 10 in each of these categories, which measures how difficult the skills in that category are for him to learn. This number is an Aptitude Value, hereafter referred to as an AV.A character with an AV of 5 in the Mind Mastery category is average at learning the skills in that category. Someone with a AV of 1 would learn those skills without even trying, while someone with a 10 would require incredible effort to learn Mind Mastery skills.
COMBAT (Armed Combat): This category includes all weapon skills, as well as armor and shield use, from medieval to futuristic.
BODY (Body Mastery): Martial Arts and athletic skills are found here.
OUTDOOR (Outdoorsmanship): Forestry, survival skills in all outdoor terrains and environments, and seamanship skills comprise this category.
AIR/SPC (Air & Space): Located here are all skills involving airplanes and other flying vehicles, space survival and navigation, and similar skills.
SCIENCE (Science & Knowledge): All forms of book knowledge, theory, and lore can be found here, ranging from astrology through social sciences to hard physics.
MIND (Mind Mastery): This category includes all skills related to magic and psionics.
ARTS (Artistry): All the creativity-based skills are here, including literature, fine arts, performing arts, and those crafts that are based on creativity rather than productivity.
CRAFTS (Craftsmanship): These skills are oriented toward making, fixing, and using mechanical devices, from woodcarving to CAD/CAM tachyon-electronic engineering.
MEDICAL (Medical Skills): First aid ranging from leeches to plastiskin, surgery of all sorts, and biological skills are found in this category.
SOC/SPY (Society & Spying): This category includes the social skills such as trading and storytelling, as well as thievery and spying skills.
These categories are sometimes referred to only by the first two letters of their names, particularly in the skills listings.
You'll be spending your remaining character points here, choosing your aptitudes. When this is done, double-check your points to make sure they add up to 100. If they don't, you might want to go back and change a stat by a few points, add one last quirk, buy some plot points, or change anything else to get the balance to exactly 100. (Remember that some campaigns may have started with some other number than 100.)
You must buy an AV in each category; you can't simply "pass" on some category. Note that one of the possible values in each category has a cost of 0. It is important to realize that you do not need a 1 to be "good at" the skills involved; a 3 is actually the standard level for "professionals" in that category's skills. For instance, a standard warrior would have an AV of 3 in COMBAT; only a weapons-master, virtually incapable of anything but combat, would have an AV of 1.
Similarly, an 8 is usually the appropriate level for someone who is no good at something. A 9 or 10 is usually reserved for those who have an exceptional mental block or are legendary for their inability to learn the associated skills. Be prepared to explain any 9s or 10s you buy.
Beware of players buying up 9s and 10s in categories they really should be buying 8s in. For example, the warrior who buys a 10 in MIND is probably going to be unwilling to even use magic items or believe in psionics, and played as such, is appropriate. But the player who buys a 10 may just say "She isn't good at magic or psionics" and try to leave it at that. That explanation warrants a 9 (spending no points). Also watch for players doing the same thing in categories that are of limited or no value in your campaign. For instance, in a science fiction world with no psionics or magic, the MIND category is pretty meaningless; you should just require everyone buy a 9 if you never intend your game to interact with any world where magic or psionics does exist, or at least require a good explanation for buying a 10.
This may be the most important step in creating your character, and the one you'll wish you could change later on (if you do it wrong) more than any other, so take care to do it well.
It is rarely necessary to flesh out the aptitudes for an NPC unless s/he will become an important part of the campaign, such as a companion of the PCs. Just write out a few of the more important values, those that will be very high or low, and don't worry about the points balancing. Remember these values when you start picking skills.