Caramel and Crystal malts are mashed uncrushed and then heat treated, leading to conversion of starches and caramelisation within the kernels.
Generally improves head retention (especially Carapils), adds to body and mouth feel.
Residual starches can lead to cloudiness. Unfermentable sugars, called dextrins, can provide sweetness. Caramelised sugars provide caramelly, nutty flavours. Heat treatment provide roasted flavours: darker kinds provide more roasted flavours.
Since starch is already converted, these malts can be used by extract brewers, or even added to the boil.
Crystal malts have crystallised sugars within the kernel.