Cyanbacteria - Sequence information (Apr/01/2010 )

I have been looking at some primer sequences for cyanobacteria and I have noticed that they contain 'R' and 'Y' see below. I work mainly with human work and I wondered if someone can explain the differences as I have never come across this.

ACGGGTGAGTAACRCGTRA
CTTCAYGYAGGCGAGTTGCAGC

I would appreciate if anyone can clarify as I am sure there is a simple explanation

Thanks in advance.

LM

they are ambiguity codes

y=c or t

r=a or g