Striking in UFC 2009 Undisputed refers to a sudden forward attack, which can be a knee, kick, punch, or even an elbow. Strikes that land use less energy than blows that miss and generally use less energy than grappling requires. A strike doesn't advance a fighter's position as can happen in grappling, unless you knock your opponent down. Standing strikes are certainly the more common and well known striking technique, but strikes can be executed from all offensive or defensive standing, clinch, or ground positions.
You can knock an opponent out with a perfectly timed strong strike if you have already softened them up a bit, with a few well placed strikes. If you hit your opponent with a series of unanswered blows that leaves them in a state where they're hands are down and they can't defend themselves, then your opponent is "rocked", and this could result in a TKO.
All the strikes in UFC 2009 Undisputed are executed using the face buttons; A, B, X, and Y if you're using a Xbox 360 or circle, square, triangle, and cross, if you're using a PS3. The various types of strikes are all executed from different positions, but these four buttons are always the ones used.