The Rs generally represent general carbon-containing chains (of length one or greater). So, in the place of the R you would have a carbon atom connected to some other things. R groups are almost never used to represent hydrogen atoms in this context. R groups also won't be used to represent heteroatoms (e.g. nitrogen, oxygen, halogens) in this context.
To help clear up your confusion, the carbonyl group is a broad category that includes both aldehydes and ketones (as well as carboxylic acids, esters, amides, etc.). In other words, aldehydes and ketones are different functional groups; however, all ketones contain a carbonyl and all aldehydes contain a carbonyl.