Diastereomer
 
Diastereomers are stereoisomers, molecules that differ only in their spatial arrangement of atoms, that are not enantiomers (1). Diastereomers have different chemical properties. There are several common types of diastereomers. Molecules containing carbon–carbon and carbon–nitrogen double bonds will exist as two different geometric diastereomers, if both of the double-bonded atoms have two different substituents. These diastereomers are differentiated with the designation cis and trans and more formally as Z and E (from the German zusammen for together and entgegen for opposite). The Z isomer describes the orientation when the two substituents with the higher priority are on the same side of the double bond. In potentially ambiguous cases, the priority of the substituents of each double-bonded atom is determined by the Cahn, Ingold, and Prelog rules (2). The other common form of diastereomers occurs when a molecule contains more than one chiral center. If two stereoisomers contain at least one corresponding chiral center with the same configuration and one corresponding chiral center with the opposite configuration, the molecules will be diastereomers. These two forms of diastereomers are shown in Figure 1.

Figure 1. The two common types of diastereomers are depicted, cis/trans geometric isomers and molecules containing more than a single chiral center.
Two atoms are referred to as diastereotopic if, on substitution with a different isotope, they would generate different diastereomers (1, 3). Diastereotopic atoms are chemically different and, importantly, can be distinguished by NMR.
References
1.  B. Testa (1982) In Stereochemistry (C. Tamm, ed.), Vol. 3, Elsevier, Amsterdam, pp. 1–48. 
2. R. S. Cahn, C. K. Ingold, and V. Prelog (1966) Angew. Chem. Int. Ed. 5, 385–415. 
3. D. Arigoni and E. L. Eliel (1969) In Stereochemistry (E. L. Eliel and N. L. Allinger, eds.), Vol. 4, Wiley-Interscience, New York, pp. 127–243.