A file of rows, columns and cells, such as Excel or Google Sheets, where data is typed in and calculated with formulas.
The spreadsheet is the world's most familiar work tool: a grid of cells where you type whatever you like and link it with formulas. Nearly every business starts with one because it is free-form, quick and needs nobody's help: a customer list, a sales log, a payroll sheet. Its strength is also its weakness: it stops no one from typing a date in the price column or deleting a row by mistake.
A spreadsheet is excellent for one person analyzing numbers and struggles when it becomes a team's operating system: several copies of the file, nobody knowing who changed what, no permissions, and formulas that break silently. Those are signs it is time for a real system. The good news is that the sheet itself is the best starting point, because its columns describe your business precisely. With Excel to App you upload the file, its columns and types are understood, and a system with roles and reports is built around them. Practical tip: clean the file before uploading: one header row, one kind of data per column, and no merged cells.