If you've decided to give Google Spreadsheets a spin, the first question (if you've used Excel at all in the past) is how to transfer your existing Excel spreadsheets to Google Spreadsheets. Fortunately, it's pretty easy.
As you'll learn in the "Importing a Spreadsheet from Excel" section of this document, it's easy to open your Excel spreadsheets from within Google Spreadsheets. All you have to do is click the File button and select the Open command, then select and upload the spreadsheet you want to import. In most cases, the Excel spreadsheet imports into Google Spreadsheets with only superficial formatting changes.
In some instances, however, Google Spreadsheets won't be able to import an Excel spreadsheet. You'll probably run into problems if you try to open a spreadsheet that has embedded charts or graphics, that include macros or pivot tables, or are overly large.
In some cases, these spreadsheets simply won't import; Google will give you a message saying that it couldn't open the file. In other cases, Google Spreadsheets will be able to open the file, but the non-Google features will be carved off. Even with fully compatible spreadsheets, it's not uncommon to find some formatting changes when you import into Google Spreadsheets; for example, the cell backgrounds might be one color in Excel and another in Google Spreadsheets. Full compatibility is just a dream at this point.
It's also possible to export from Google Spreadsheets back into Excel's XLS format. This lets you work on your spreadsheets online, for sharing and collaboration, but then go offline with an Excel file for more private or detailed work. Just click the File button and select Download as XLS; this downloads an XLS-format version of the spreadsheet file to your computer.