Because they are bacteria, they are quite small and usually unicellular, though they often grow in colonies large enough to see. They have the distinction of being the oldest known fossils, more than 3.5 billion years old, in fact! It may surprise you then to know that the cyanobacteria are still around; they are one of the largest and most important groups of bacteria on earth. Some can form rocky outcrops called stromatolites, the most famous of these are those found in shark bay in Australia.

Cyanobacteria can be found in almost every terrestrial and aquatic habitat—oceans, fresh water, damp soil, temporarily moistened rocks in deserts, bare rock and soil - and even Antarctic rocks. They can occur as planktonic cells or form phototrophic biofilms (slimy skin on rocks!). A few are endosymbionts (inside) in lichens.

Uses suns energy for photosynthesis

Commonly known as blue-green algae and they are photosynthetic (producer) they are not algae. They are a phylum of ancient bacteria. They have existed for 2.5 billion to 2 billion years and are the only bacteria known to have a circadian clock (internal biological clock) which controls when their cells grow and divide. Many cyanobacteria form motile filaments of cells, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than in the vegetative state, and the cells on either end of the motile chain may be tapered. In order to break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, called a necridium. The photo is of the marine Cyanobacteria Rivularia bullata, from Karaka Bay, Great Barrier Island and was taken by Dr Mike Wilcox.

http://www.emsl.pnl.gov/science/membrane/cyanobacteria.jsp (accessed 17/04/13) Scheil et al Guide to common intertidal species of the South Island, N. Z.
Rivularia bullata Photo credit Mike Wilcox