Cyanobacteria are a division of organisms in the domain bacteria that are autotrophic by means of photosynthesis. They are commonly found in freshwater aquatic environments, but they can be found in all sorts of places such as kitchen drains, soil, plant pots, walls of greenhouses, and symbiotically linked to fungi to form lichen.
As their classification dictates, cyanobacteria are prokaryotic organisms lacking a true nucleus and membrane bound organelles such as ribosomes. All of them are unicellular, but some connect with sheaths to form colonies or filaments. Sometimes, individuals will be seen specializing into different forms, most notably into nitrogen fixing cells known as heterocysts. Thusly, cyanobacteria play important roles in the nitrogen cycle, allowing plants to use nitrogen by taking it from its unusable form in the atmosphere and converting it to such useful chemicals as ammonia, nitrate and nitrogen dioxide. The walls of cyanobacteria always stain gram negative and are described as 'gelatinous'.
These ancient creatures can be found in fossil form from as long as 3.5 billion years ago. They've formed many Proterozoic oil deposits and have left quite a few stromatolites for our enjoyment. They were the organisms largely responsible for the release of large quantities of oxygen into our atmosphere by photosynthesis, a fact which many aerobic organisms such as ourselves are (not always) thankful for.
Cyanobacteria have been called "blue-green algae" because they are seen in similar environments and look superficially similar to real algae. However, algae aren't even close to cyanobacteria and are classified under the kingdom protista. Interestingly enough, cyanobacteria are believed to be the ancestors of chloroplasts that now inhabit eukaryotic photolithoautotrophs such as plants and algae.
Although most cyanobacteria are bluish green, some can be gray-brown or bright green. The first two use chlorophyll a with phycobilins as their pigments, but the later also has chlorophyll b and lacks phycobilins.
Some cultures use cyanobacteria as a food source. Spirulina, being high in protein, was used by the Aztecs and is still cultivated in ponds by people living with a tropical climate. It's also been turned into a health food by longevity obsessed Americans that supposedly boosts your immune system and prevents cancer. However, other species are harmful and will poison livestock and pets who drink out of contaminated bodies of water. Also, such species as Lyngbya majuscula cause skin and eye irritation to swimmers that are unfortunate enough to come in contact with it.