The world of microbiology is a fascinating universe. There are thousands of different species of life-forms that can only be seen with the aid of a microscope. Even with a microscope, these tiny creatures are so foreign looking that they could easily be confused with some space alien from a science fiction movie. There are dozens of types of these creatures - one type of which is called the Protozoans. Within the protozoans are a group of single-celled creatures known as the amoeboids (or sometimes just amoebas).
All amoeba are single-celled organisms. They can often be found in groups, but they live quite well on their own, despite having only one cell. They found in water environments as the delicate outer layer of the cell must be in water in order to not dry out and kill the amoeba.
Amoeba have an interesting way of moving about. They always live in water, but they don't "swim" in the same sense that a more complex organism does. Instead, an amoeba moves by extending part of it's outer membrane - forming a temporary leg called a pseudopod. These pseudopods are like temporary arms and legs, and allow the amoeba to creep about in search of food.
The microfilaments that are contained inside the amoeba are vital to making these pseudopods. It is estimated that about half of the internal space in an amoeba contains these microfilaments - not leaving a lot of space for other "organs".
Once an amoeba finds something interesting to eat, it's method of consuming that food is also somewhat unique. They "eat" by literally engulfing the target. The amoeba will flex its membrane around the food, enveloping it by extending the membrane until the food is entirely inside the organism.
Waste, water, oxygen, and other chemicals that are needed by the amoeba are transferred in and out of the amoeba through the membrane shell of the organism. There are several sub-types of amoebas, but they all use their membrane to communicate with the outside world. All amoeba live in water, so there is never a lack of water, but getting waste out and other nutrients in is a major part of what an amoeba does with it's day.
Amoebas replicate by cell division. They get physically bigger to a point. Once the maximum size is reached, they duplicated much of their internal structures and genetic material, then they literally split in to two identical new amoebas. Amoebas do not have chromosomes in the same way that more advanced life does.
As I mentioned before, there are dozens of sub-types of amoebas. They are generally split in to categories based on how they make pseudopods. A detailed listing of the sub-types is not really possible in this small space - and let's face it, a small blob is just another small blob to anyone who isn't a microbiologist (who aren't likely to be using this overview as a resource in the first place).