A DNA microarray has thousands of small dots of known DNA sequences that are printed in a gridlike array. A small-molecule microarray has dots of known small molecules rather than dots of DNA sequences. Scientsts use custom-built microarrayers which transfer tiny amounts of DNA solution or small molecules to a glass slide.
Also featured on the HHMI DVD Scanning Life's Matrix: Genes, Proteins, and Small Molecules. Available free from HHMI.