Tandem repeat

Tandem repeats occur in DNA when a pattern of nucleotides is repeated. The repeats sit next to each other, in tandem. These are multiple copies of the same base-pair sequence lying end-to-end. 
An example would be: ATTCG ATTCG ATTCG in which the sequence ATTCG is repeated three times.
Terminology.
If between 10 and 60 nucleotides are repeated, it is called a minisatellite. Those with fewer are known as microsatellites or 'short tandem repeats'.
When exactly two nucleotides are repeated, it is called a "dinucleotide repeat" (for example: ACACACAC…).
When three nucleotides are repeated, it is called a "trinucleotide repeat" (for example: CAGCAGCAGCAG…). Abnormalities in such regions can give rise to trinucleotide repeat disorders.
The repeat unit copy number may be variable. This is a variable number tandem repeat (VNTR). 
Uses.
A tandem repeat pattern helps determine an individual's inherited traits. Tandem repeats can be very useful in determining parentage. Short tandem repeats are used for certain genealogical DNA tests.
In genetic fingerprinting and DNA profiling, DNA is examined from tandem repeats in the chromosomal DNA.