Lichen is a combination of fungi and algae. This organism is very important for the terraformation of Mars because they can survive very adverse conditions, just like those on a partially terraformed Mars. Lichen uses the light energy in sunlight to turn carbon dioxide and water into sugars and oxygen just like every other plant.
Although lichen can withstand very adverse conditions, the current Mars would not even be able to sustain even the hardiest lichen. The temperatures in the tropical regions would be high enough to support lichen, but there is nearly no water, so the lichen would quickly die. The water problem could be solved by artificial irrigation but there is not enough carbon dioxide in the atmosphere to make enough oxygen for human life.
Partial terraformation is the name given to the point in the terraforming process when the temperature is high enough to begin releasing water and carbon dioxide can be released from the ice caps and regolith. After the temperature is raised by a small amount, enough carbon dioxide from the ice caps and the regolith will be released to keep the temperature rising even further.
Once partial terraformation is complete, there will be a large enough increase in temperatures to melt the underground permafrost. Since the water vapour is another greenhouse gas, the process continues and the temperature continues to rise and there will be enough water to support lichen.
The main disadvantage of lichen is that they grow and reproduce very slowly. Genetic engineering would probably be able to speed up the process but they would only be capable of surviving in conditions well suited to them due to the engineering. The second disadvantage is not exclusive to lichen, but rather to all plants in general. They are relatively inefficient, even the most efficient plants can only convert 3.5% of the Sunís energy into sugar and oxygen. Therefore, it will take a long time to convert the carbon dioxide in the atmosphere into oxygen.
Lichen is clearly not the quickest way to go, but it is the easiest, most cost effective and the most technologically feasible.