To tabulate information is to organize it into a table. If your friend borrows money from you every week, you might want to tabulate what she owes you.
Tabulating is a way of processing information or data by putting it in a table. This doesn't mean the kind of table you eat off of, though. It refers to a table, or chart, with rows and columns. When tabulating, you might have to make calculations. Someone who enjoys tabulating financial information might make a good accountant.