Before collecting the data, did you know which group would be larger if
there was a difference? If so, then you can use tails=1. Otherwise
(the difference could have been in either direction) you use tails=2.
This is discussed in more detail in any introductory statistics book.

Help on TTEST seems quite clear on type, assuming that you have a
background comparable to having covered t-tests in an introductory
statistics course. The choice is based on how your experiment was
designed, and involves more nuances than are easily addressed in a
simple newsgroup reply.

Jerry

goodmonkey wrote:

Please define 'tails' and 'type' better than the TTEST help file. Online
statistics glossarys are of little use.

thanks.