Having been in the audience for a few shows, there is often a few minutes before recording begins where the warm-up act or the producer ask the audience to perform different types of laughs and clapping. These are usually to cover the recording edits (where material has been edited out for timing or even legal reasons), so that the sound at the edit doesn't jar when viewing the finished programme.
The audience perform a range from a soft chuckle to a belly laugh and all combinations inbetween. These are usually on the tape for that show for the editor to use, but I would imagine some may be kept for a series.
If you ever watch old editions of The Two Ronnies, there is a woman's laugh that crops up constantly.
Last of the Summer Wine, although shown to an audience to record their laughter, will often have additional laughter dubbed on if the director feels the audience didn't get a gag or if it will help a comic performance.