A codon is a triplet of bases (or nucleotides) in the DNA coding for one amino acid. The relation between codons and amino acids is given by the genetic code.
The triplet of bases that is complementary to a codon is called an anti-codon; conventionally, the triplet in the mRNA is called the codon and the triplet in the tRNA is called the anti-codon.
Table: the genetic code. The code is here expressed for mRNA. Each triplet encodes one amino acid (notice three triplets are "stop" codons which signal the end of a gene.