when to use & pros/cons of dataframe vs. matrix?

Anika -- these are good questions and many on the list could  
expatiate on them.  These erudite people are also busy, however, and  
that is why the R-news posting guide suggests that you study an  
introductory book on R before asking general questions.

When "should" I use a dataframe vs. a matrix?  What are the pros  
and cons?
If I have data of all the same type, am I usually better off using a
matrix and not a dataframe?
What are the advantages if any of using a dataframe vs. a matrix?
(rownames and column names perhaps?)

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