Aptamers (from the Latin aptus – fit, and Greek meros – part) are oligonucleotide or peptide molecules that bind to a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool, but natural aptamers also exist in riboswitches. Aptamers can be used for both basic research and clinical purposes as macromolecular drugs. Aptamers can be combined with ribozymes to self-cleave in the presence of their target molecule. These compound molecules have additional research, industrial and clinical applications.
More specifically, aptamers can be classified as

DNA or RNA or XNA aptamers. They consist of (usually short) strands of oligonucleotides.
Peptide aptamers. They consist of one (or more) short variable peptide domains, attached at both ends to a protein scaffold.