coopster - 8:18 pm on Jan 21, 2011 (gmt 0)
The first is a multiple-column index, compound index, compound key ... many different names/aliases. MySQL refers to it as a multiple-column index. The information in your table would be sorted by the value in section_ID and then by tag.
The second is a single-column index, sorted by that column specifically with no specific sort on any secondary column(s).