When you use the subtotal wizard, your data is grouped, and you can use buttons to expand and collapse your group:
Read more about subtotals here
You can also group data in Excel without using the subtotal wizard. I had an occasion to do this just the other day, in fact.
All you do is highlight the rows of data that should be grouped together. Then, choose:
Data-->Group and Outline-->Group
Repeat as necessary for the different data groups you want to create.
Why would you want to do this? I'm not sure. But, I did it to separate the top 90% of my purchased items from everything else. I could have assigned them all an ABC Code (A=top 90%, B=90-95%, C=95-100%) and added subtotals, but I didn't really want to do that, and I didn't need the subtotals. This was quick, easy, and very functional in this application.
Pretty Neat Trick!