Basically a table containing rows and columns of figures, which can be set to carry out mathematical functions automatically.
If you have a column of 20 different numbers with a total at the bottom, for instance, when you add a new number half way down the column the total should alter to include the new number.
You can add pictures (and sounds, by jingo) to a spreadsheet. You can write a few lines of text, do fancy headings, colour cells and borders and so on. You can make a simple but effective database.
Excel and Lotus 1-2-3 are the best known spreadsheets.