Liners are added extras to nappies, they don't usually come with them
They can be made of various materials, some are disposable in the bin, some are flushable (fab for weaning poo!) and some are made of fleece or Coolmax (so washable with the nappy).
They go between the inner of the nappy and baby's skin. So if your nappy is an AIO, it'll lie on top of the top insert. If it's a pocket, it goes erm... well, between the inside of the nappy and baby's skin
We use disposable liners during the day, mainly because my LO is in childcare and flushable liners makes things easier. We use fleece at night for the "stay dry" factor.
With disposable liners, whilst it can help draw moisture away, the main thing for us is that it means you can use a barrier cream without having it get onto the nappies (which can stop them absorbing). We use the Organic Baby cream by The Green People (cheapest I've found is at
www.littlepants.co.uk). It's the best we've found, both for clearing up nappy rash and also as a barrier to stop it coming back. The likes of bepanthen, sudocrem, metanium etc did nothing at all for us.
So anyways, short answer... Liner + barrier cream is the way we've managed to beat nappy rash