He was described as having a short nose and protruding eyes. Much of what we know of him comes from Plato.
The TheŠtetus is a dialogue by Plato thought to have taken place in the year 369 B.C. In this dialogue Socrates, Theodorus and Theaetetus try to define what knowledge is. Other participants in the dialogue are Eucleides and Terpsion.
Although the dialogue never succeeds in giving a clearcut answer to the question "What is knowledge?", it shows the reader some failed and some more fruitful approaches to the question. Approaches not very different from these are still discussed in modern epistemology.
Knowledge is perception
(More about the different approaches...)
Knowledge is true belief
Knowledge is justified true belief
The full text is available from Project Gutenberg