Specific Range? Macros?... I don't know what area I am looking for this answer in....

So, I have created a workbook that I am trying to get right. My question is, if I want to only be able to put data into a specific range( say A4 to M16) and I don't want to be able to enter data in anything outside of that area. How would I keep someone from doing that. Essentially, I want to start at A4 and be able to TAB all the way over to M4, and then instead of entering down to A5, I want the spreadsheet to "know" that I don't want data in N4 and instead TAB over to A5. I know this works, because I use a spreadsheet that does it every day. Where should I look for the answer, by recording a Macro? or by locking a portion of the spreadsheet? Everything I have tried doesn't work. So, I am sure I am screwing up somewhere. Thanks for any help.