I love my flushable diaper liners and wouldn't be without them. Diaper liners are not a necessity, and they're not to everyone's liking, but I personally would not be without them.
Flushable liners come in packs of individual sheets or on a roll similar to a toilet paper roll (but don't try to use toilet paper in place of a diaper liner, it will just disintegrate and leave a huge mess). You pull of one liner each time you change your baby, and place it between the diaper and your baby's skin. The diaper liner will catch any solids, so when you change your baby, you simply have to lift up the flushable liner and toss it in the toilet (where solids belong).
Diaper liners help to protect your diapers from stains from creams you might use on your baby's bottom and stains from bowel movements. Also, because almost all of the solids end up on the liner, there's no need to dunk, spray or scrape your baby's soiled diapers.
Diaper liners do not provide extra absorbency. If you require additional absorbency, you'll want to consider using diaper doublers. You can, by the way, use diaper liners and doubler together if you want extra absorbency plus easier clean up.
A package/roll of flushable liners costs approximately $10 for 100 sheets.