Phytoplankton are microscopic plant life free floating in water bodies across the world. If you've ever looked at satellite images of Earth, and noticed long strips of green patches in the ocean, then it's most likely a bloom of Phytoplankton, as seen in the picture below.
Phytoplankton are literally the backbone of this planet - they produce an estimated 50% of the worlds oxygen, as they breathe CO2 and leave Oxygen as a waste product through the process known as photosynthesis. There is not a single organism on this planet which can match the effect of Phytoplankton on the sustained life of this planet, nor an organism which could cause such a cataclysmic effect if their population size reaches critically low levels. A Phytoplankton's ability to convert sunlight, warmth, water and minerals into protein, carbohydrates, vitamins and amino acids was the very basis for life to exist on Earth, and still are.
Phytoplankton are also the the basis of marine food webs, the first trophic level. Whales and other filter marine creatures consume Plankton as the primary source of nutrition in their diets.
Giving a balance of views, Phytoplankton do have drawbacks. Large blooms can suffocate marine life in areas - vast coverage can block out sunlight, affecting marine life. Dead Phytoplankton sink to the bottom of the ocean and the decomposers (bacteria) absorb oxygen, reducing the amount of oxygen available in the water, essentially suffocating all other lifeforms in the vicinity.
On a global scale, they act as CO2 regulators - through photosynthesis, they remove CO2 from the atmosphere, regulating planetary temperatures, and creating a carbon sink.
Unfortunately, levels of Phytoplankton have steadily declined since the 1950's by approximately 1% per year, mostly through the theorised climate change. Considering these organisms are the foundation of all lifeforms on this planet, it's pretty worrying.