Applying a Cell Border
From Help Wiki
Applying a cell border is a quick and simple way to form a small graph or draw attention to an area of your spreadsheet in particular. These borders are easily customized and fit into any Excel spreadsheet.
Cell borders come in many varieties and can be easily experimented with without disrupting the spreadsheet itself. In Excel 2007 on Windows, the option for cell borders can be found under the Font menu. There are many styles of borders from one-sided to all-sided.