Definitions

(The following are definitions I've encountered while reading Trace Your Roots with DNA: Using Genetic Tests to Explore Your Family Tree by Megan Smolenyak Smolenyak and Ann Turner [Rodale, 2004; ISBN 1-59486-006-8]. These appear in blog posts--often without further explanation. Let me know if you need more definitions.)
Allele
"One of the alternative versions of a gene or genetic marker that can exist in a particular location on a chromosome." -- Trace Your Roots with DNA, p. 249.
Haplotype
An individual collection of "short tandem repeat" (STR) mutations within a DNA segment. Individuals with matching (or nearly matching) haplotypes belong to the same haplogroup. They likely share the same MRCA.
Marker
"A distinctive landmark that occurs in an otherwise featureless stretch of DNA; a DNA sequence with known genetic characteristics that can be tested for purposes of comparison." -- Trace Your Roots with DNA, p. 253.
MRCA
Most Recent Common Ancestor. The nearest common (male--in the case of a Y chromosome test) ancestor shared by two related individuals (cousins/siblings/etc.).
Patrilineal ancestry
A person's ancestral line consisting exclusively of fathers: me, my father, his father, his father, ... etc. (patri = "father's"; lineal = "line")
Y-DNA
Y Chromosome DNA test. A popular genealogical DNA test for matches (or near matches) of "markers" that appear on the Y chromosome. Because only males have a Y chromosome, the test determines if two living males--generally with the same surname--share the same MRCA (i.e. whether they have common patrilineal ancestry).