A spreadsheet application is a computer program such as Excel, OpenOffice Calc, or Google Docs Spreadsheets.
It has a number of built in features and tools, such as functions, formulas, charts, and data analysis tools that make it easier to work with large amounts of data.
The term is also used to refer to the computer file created by the above mentioned programs.
In this sense, a spreadsheet is a file used to store various types of data.
The basic storage unit for a spreadsheet file is a table.
In Excel an individual spreadsheet file is referred to as a workbook.
A term related to this is worksheet, which, in Excel, refers to a single page or sheet in a workbook. By default, Excel has three worksheets per workbook.
So, to put it all together, a spreadsheet program, such as Excel, is used to create workbook files that contain one or more worksheets containing data.