Excel is a spreadsheet program, useful if you have lots of numerical or financial data to view or analyse.
Access is a database, useful if you have a lot of data that you wish to break down or report on. In some ways their jobs overlap, but the main differential is that Excel is fantastic if you need to do calculations or graph your data.
I use both, Access is used to store the data my office collects daily. This data is entered into Access forms, where it's collected in tables. The data is then queried and viewed in reports. If any further calulation of analysis is required the report is output into Excel.
I always find Dummies books useful for getting to grips with a program, as, whilst the authors are obviously very knowledgable on their subject, they use very little technical jargon.