Hi Nicola,
There isn't a yes /no answer to your question as it all depends on which format your file is in and which format your particular ereader will read. If they are the same then no problem, if they are different then you will need to convert the file into the correct format for your reader. Calibre is a good tool for converting files from one format to another. Kindles use the .mobi format.
When you get a book there should three files, a picture of the front cover usually in .pdf format, the book itself in whatever format, and a metadata file which contains book details such as ISBN, publisher, publishing date and a textual description of the book. When you select a book in Calibre it will display the cover at the top and the metadata underneath. Editting metadata can be tricky and isn't something that the casual book reader should attempt (or need to do)
Hope this helps.