Although DNA sequences are represented by four nucleotide bases (A,T,G,C) they can be represented by alternate letters based on different scenario. One of the example would be sequencing data, where N is used if nucleotide base can not be indentified for a particular position. It can also be used to represent motif sequences.
The picture below represents the alternate representation for possible nucleotide bases at given position. Image taken from here.
Here is another link, that talk into more details regarding this.