Buddha is an "Enlightened" teacher, a state of being, and alternately an individual religious figure in Buddhism.
The Enlightened One
The term Buddha means "Enlightened One" in Sanskrit, and refers to any of a large number of past and future beings (usually human) who achieve Nirvana after their death. Depending on the type of Buddhism, the number of possible Buddhas vary from 35 (Tibetan), to as many humans as are on the planet (Mahayana).
Two "famous" Buddha images include the image of Anitaba Buddha (the first Buddha, Gautama Buddha) who is depicted as thin, contemplative, and suffering. The second is the "Happy Buddha" or "Laughing Buddha" (in the US, often seen at Chinese restaurants) who is the coming Buddha, who will bring joy and luck to all.
There even was a female Buddhas portrayed in Buddhism, Kwan Yin.
The State of Being
A Buddha can also refer to a person who has achieved "Buddhahood", which is a point where you break the cycle of endless lives of reincarnation, cease to have a self, and become part of the whole of creation upon your death. The concept could be looked at as "Upon your death, the grasses use your body for nutrition, so you become part of the grasses, which are then eaten by the goats making you part of the goats, which are themselves eaten upon their death... and so on, until that which "made" you, has, over time, touched all things in creation. Then, expand that simplistic materialistic view to a spiritual level as well. This is Buddhahood.
Buddha as the Founder of Buddhism
Finally, Buddha can also refer to Gautama Buddha, the teacher and founder of Buddhism. According to tradition, being a member of the rich society, he was unaware of the suffering of people until around the age 13, he took a trip into the city and saw starvation, suffering, poverty and death for the first time. This prompted him to leave his life of luxury for an ascetic life. In his meditations, he became enlightened about how to live on this world and ultimately escape that endless suffering. His writings, the Sutra, become the foundations of the beliefs of Buddhism.