A Power is a mechanism by which a character's impacts the world in a way that is unique to them. The energy that drives these powers is drawn from that character's Power source.
At each power's core is one primary effect and zero or more secondary effects. Each power source grants access to a number effects, some of which may be unique to that source. Powers have properties, including duration, number of targets, and range. These properties are inherited from the primary effect, and may be modified by a secondary effect.
Power Points, or PP, are a resource that is spent to manifest a power. Every effect has a base cost, normally 0 to 2 PP. More Power Points may be spent to improve the primary effect, or introduce secondary effects.
Each character has a pool of Power Points. This pool starts at 25, and increases as the character gains levels, according to the class, including their first level.
A single power is limited in how many Power Points can be invested in it, by a character's Power Point Limit. This is normally equal to the character's total HD plus 1. Powers with an Action Type of Standard may be manifested as a Full Round action to increase the PP Limit by 2, and many classes have ways of increasing the PP Limit of specific effects.
Simple powers are relatively easy to create, assuming you can add small numbers. Increasing damage, range, the number of targets, etc, can be done on the fly. However, introducing secondary effects or calculating complex class feature interactions can take some time. Therefore, it is heavily suggested that each player comes up with a number of pre-calculated powers that they can pull out of the hat. These could be considered signature moves, and it is encouraged that each player come up with names for their favourite powers. This is especially important as levels increase, as the increased PP limit leads to an increase in complexity.
Of course, this shouldn't discourage improvisation where appropriate. However, both the players and the GM should be careful to ensure that such improvisation doesn't bog down combat; keep improvisations simple.
Constructing a Power
Peter Chorster is a 3rd level Paladin Mantlebearer. Being level 3, he has 3 HD, so his Power Point Limit is 4. He decides to create a Smite-based power, which has a base cost of 1. He spends 2 additional points to increase the damage by 1[W], and 1 additional point to introduce the Heal Secondary Effect. This new power now attacks the AAC of one adjacent enemy for 2[W] damage with advantage, and heals Peter for 1d6 if he successfully hits.
Each power has a list of descriptors which are used to define which feats, equipment effects, buffs, etc. may affect that power. The descriptors are: