I took a look at Michael Kofler's book "Definitive Guide to Excel VBA" regarding this.
Not a whole lot of difference. Both methods are used without parameters and do not return a direct result. They change properties such as ActiveCell and Selection.
Activate is generally intended for individual cells, and Select for ranges of cells (these methods also are used with other objects, such as worksheets, charts, etc, but my response here is to speak in general terms).
Whether you Select range A1:A5, or Activate A1:A5, A1 will be the active cell, and A1:A5 the selected range.
The only other thing (which really speaks to their same-ness instead of difference) is that both methods can only select cells or ranges in the active worksheet. If you are on Sheet1 and try something like Range(Sheet2!A1).Select, you'd get an error.
Bottom line, not a heckuva lotta diff, especially where cells and ranges are concerned.