Wikipedia defines tautology well

In grammar, a tautology (from Greek tauto, “the same” and logos, “word/idea”) is an unnecessary repetition of meaning, using more than one word effectively to say the same thing (often originally from different languages). It is considered a fault of style and was defined by A Dictionary of Modern English Usage (Fowler) as “saying the same thing twice”, when it is not apparently necessary to repeat the entire meaning of a phrase. “Fatal murder” is an example of a tautology. If a part of the meaning is repeated in such a way that it appears as unintentional, or clumsy, then it may be described as tautological. On the other hand, a repetition of meaning that improves the style of a piece of speech or writing is not necessarily tautological.

 Here are some example sentences:

• He went there personally.
• The evening sunset was golden.
• My first priority is to quit smoking.
• There is a lot of frozen ice in winter.
• I heard the story with my own ears.
• Scientists like making predictions about the future.
• The hospital is in close proximity to the school.
• The Sahara is a very dry desert.
• In my opinion, I think she is right.
• Having an eye test is a necessary requirement for passing the driving test.
• She is a dark-haired brunette.
• He wrote an autobiography of his own life.