Complex sentence: Peptoids, or poly- "N"-substituted glycines, are a class of peptidomimetics whose side chains are appended to the nitrogen atom of the peptide backbone, rather than to the α-carbons (as they are in amino acids).
Split into simpler sentences: Peptoids, or "N"-substituted glycines, are a specific subclass of peptidomimetics. They are closely related to their natural peptide counterparts, but differ chemically in that their side chains are appended to nitrogen atoms along the molecule's backbone, rather than to the α-carbons (as they are in amino acids).