Each primarily have their own purposes: Word for Word Processing, Excel for Spreadsheets and Access for Databases. Whichever of those 3 things you want to do will determine which application is best for the job. However, in each case a lot of what you would do with the primary application can be done in the secondary ones, though not as efficiently. For example it is possible to do formulas similar to Excel in Word. It is possible to do a lot of database activities in Excel. It is also possible to do things in one application and bring them into another. A table of data in Excel can be copied and pasted into Word, becoming a Word table, or save and exported so that it can be imported into Access and become a data table. You can also set up links between different documents. So you could link a Word document to figures in an Excel document, so that when the Word document is opened, it will show the up to date figures in the Excel document. These are just a few examples of what you can do. There are many others. Sometimes that interchangeability is necessary, as it is more efficient to do some of the work in the appropriate application and then bring it into one of the others. Of the 3, Excel is probably the most flexible of them and has the widest range of things it can do that can be done in the others, and Word would be the most limited.