There are a number of Computer Science and GIS glossaries on the Web.

N.B. A problem for GIS students and professionals is that the terminology of GIS is not fixed. What is an 'arc' in one person's vocabulary may be a 'segment' in another's and an 'edge' in another's. Do not be surprised if the glossaries do not always agree!

On the other hand, looking up terms in a glossary can be an efficient way of clearing up misunderstandings.

AGI Dictionary	The Association for Geographical Information's (AGI) searchable dictionary of 980 GIS terms.
Computing Dictionary	The 'Free Online Dictionary of Computing'.
ESRI Library - GIS Glossary	N.B. Downloadable as a PDF document. Provides a useful, if ESRI-oriented, set of GIS terms.
GITA	Glossary of GIS terms.
PC Webopaedia	Another online computer science glossary site.
whatis.com	The 'Whatis.com' site provides a Computer Science glossary of terms. Some of the descriptions run to a page or more. You can learn a lot by searching this site.