In a new worksheet, let's simulate a form. To do so, first select any four discontiguous cells...that is, four cells that don't touch each other.
For example, select some cell near the top right corner of your sheet. Hold down your Ctrl key then click on a cell several rows below it. With Ctrl still down, click on a cell several rows below that. Then with Ctrl still down, click on some other cell near the bottom.
These are the cells where data will be entered in our simulated form.
With these four cells selected, use the Fill Color icon (or choose Format, Cells, Patterns) to assign a cell shading to them. These colors won't affect the solution; they merely allow us to see what's happening.
With these four cells still selected, choose Format, Cells. In the Protection tab, you'll see that the cells are locked. Unlock them by clicking the check box to uncheck them. Then choose OK.
The last step is to protect your worksheet. Choose Tools, Protection, Protect Sheet. Then choose OK.
Now, when you press Tab, Excel will jump from colored cell to colored cell, working from top to bottom, then jumping to the top again. That is, Excel tabs among the cells in your simulated form. To return to the original condition, choose Tools, Protection, Unprotect Sheet.
By the way, you might take the time explore the Protect Sheet dialog. It allows you to specify how seriously you want to protect the locked cells. These settings might be useful, depending on what your are trying to achieve.