Grid tabs allow you to store row and column data, somewhat like an Excel spreadsheet. However, the columns are set by an administrator, and cannot change.
Also, administrators are able to setup validation rules for required fields, numeric fields, etc.

Most popular grids:

List - Includes data for any type of list; Product, SKU, Component, etc.

Timesheet - Includes project/user data that includes dates, estimates, and actual time for a particular task.

Budget - Includes collective project costs per item, cost center, vendor.  List can also total your estimated vs. actuals.

Like all other tabs, grid tabs display their data and functionality inside the “Project Data” part of the screen. Working downwards from the top of this area, we start with the grid
toolbar. The grid toolbar contains buttons and other controls that let you work with the data stored in this tab.

Moving downward from there, we have the grid itself. This part of the screen displays the row and column data (a “grid” of fields).

Below the grid is the pagination toolbar. This toolbar allows you to page through large record sets. The next/previous buttons will probably look familiar to you. There is also a refresh button, allowing you to refresh the current page. Grid tabs display up to 25 rows per page by default.