History of HO/POs
Hammer Ons and Pull Offs (HO/POs) first appeared in Guitar Hero I. All the notes in Guitar Hero are depicted as colored circles, corresponding to the different fret buttons on the guitar controller. In the middle of the colored note is a white circle. Regular notes have a black circle around this white circle, while Hammer Ons and Pull Offs don't. They only appear in guitar or bass parts.
The HO/POs in Guitar Hero I were known for being extremely difficult to master, as you had to press the correct fret button on basically the EXACT frame that the note crossed the strum line. Regular notes had a much larger "timing window," meaning you could strum the notes slightly before or after they crossed the strum line. Guitar Hero II improved the HO/PO system greatly by increasing the timing window for when you could press the fret button of a HO/PO note. There was even a message that would appear in the loading screens that said that HO/POs "really work this time!" This same system was also used in Guitar Hero: Rocks the 80s.
Guitar Hero III had a drastically different HO/PO system. The timing window for HO/POs was increased so much, you could press and hold the fret for an upcoming HO/PO immediately after you strummed the regular note in front of it. Even if the HO/PO note was measures later. This simple change made Guitar Hero III as a whole, easier than the other games in the series on the harder difficulties. This is generally considered to be the case because the developer of the game changed from Harmonix to Neversoft. Guitar Hero: Aerosmith uses a HO/PO system similar to Guitar Hero III.
Rock Band also uses HO/POs. Regular notes in Rock Band are depicted as rectangles, and HO/PO notes are depicted as somewhat smaller rectangles. Since they look so similar, many people have a harder time seeing HO/POs in Rock Band than they do in Guitar Hero. The timing window for Rock Band HO/POs is very similar to the window in Guitar Hero II, possibly because Harmonix moved on from Guitar Hero II to develop Rock Band.
The Guitar Hero series has continued to refine their HO/PO system, starting with Guitar Hero: World Tour. The timing window is much more similar to Guitar Hero II's, giving the system a more realistic feel. Guitar Hero 5 probably has the best refinement of the system in the Guitar Hero franchise. Guitar Hero: Metallica and Guitar Hero: Van Halen use Guitar Hero: World Tour's system, while Band Hero uses Guitar Hero 5's system.
Rock Band 2 also improved upon Rock Band's HO/PO system, mainly by redesigning the notes that depict HO/POs so that they can be seen much more easily. The HO/PO notes are still smaller rectangles, but they have a double border, which gives them a more distinct look from the regular rectangular notes. The Beatles: Rock Band also used these redesigned notes, as did Green Day: Rock Band.
There is no information yet how the HO/PO systems will work in Rock Band 3 or Guitar Hero 6.