A PVR is a receiver with a built in hard disk. It works just like a normal set top tob, except that it enables you to make recordings of TV programs onto the hard disk. You can watch these recordings at a later date. You can usually pause or rewind live TV (useful if you are interrupted during your favourite programme).
The primary use is for timeshifting programs. With most PVRs you can set repeating timers (to record the same program each day or each week). Some allow you to set search timers which find programs with certain names and will then record them for you. Search timers can be very useful, allowing you to specify which programs you want recorded. This way your PVR will happily fill your hard disk with recordings for you to watch when you want.
PVRs are less suitable for making permanent recordings of programs you want to keep since the hard disk will eventually fill up. Some PVRs allow you to connect your PC via a USB lead, copy recordings onto your PC from where you can burn them onto a DVD, but this is can be a labourious process. If making permanent recordings is your main aim, a DVD recorder with a built in Freeview tuner would probably be more suitable.
Pause/rewind live TV
Most PVRs have the ability to pause live TV, recording onto the hard disk until you start playing again. This can be useful to create your own breaks in long programmes or if you are interrupted. Some PVRs constantly record the live TV you have just watched, allowing you to rewind live TV if you missed something or want to see it again.
Find Out What's On TV
An EPG (electronic programme guide) is a TV listings program built into your Freeview receiver. It lists the channels availble and programs that are on each. Most PVRs come with a 7 day EPG, although some can have 14 day EPGs. From the EPG you should be able to press a single button to set a timer to record a programme.
Never Watch Live TV Again!
Once you've experienced a Freeview PVR you may never want to watch live TV again. I love my PVR. I use search timers to record all the programmes I like. I can then watch them when I want to, not when they are being broadcast. It is very rare that I watch live TV at home any more. The main downside to this is that I have to avoid office discussions of last nights TV.