Spreadsheets are programs that display data in grids of cells: columns and rows help you organise it.
Cells can only have one type of data entered into them.
Cells can have formulas in them which do calculations using data in other cells, e.g. adding up or averaging columns of numbers.
Formulas can use references to cells that are relative (will change if moved to another cell) or absolute (fixed).
Spreadhseets can analyse data to display graphs, charts and tables.
They can be used to model real life situations, such as how much profit a company can expect to make.
Spreadsheets can import or export data to other programs, with the CSV format being quite popular for this purpose.