1) Excel is predominantly a statistical tool, suitable for analysis and such like, alongside the presentation of such a context of material through its use of 'cells', little squares that are defined by their column (A, B, C, AA etc) and their row number (1,2,3,150 etc). Conversely, Word is a typewriting tool, suitable for laying out typed documents in a well presented way, as well as providing various tools and formatting options to allow a document to be created and viewed. Essentially they are both quite different programs, but being part of the MS Office suite, enjoy a large level of compatability in terms of displaying spreadsheets as part of word documents. The general rule should be that if you're trying to do something 'Maths' wise then use excel, if you're trying to do something language wise, then use word.
2) A cell is a square on a worksheet that is defined by its co-ordinate, such as A1, C53 etc. In versions prior to excel 2007, each worksheet has 256 columns and 256 ^2 rows, each pointing to a unique cell. These cells can be resized and have various formatting options performed to display data. This collection of cells forms a worksheet, of which many can exist in a workbook. A workbook is a collection of worksheets, and a worksheet is a collection of cells.
It is not possible to colour the tabs (as far as i'm aware), unless you use a hack that uses API calls to alter system colours. Not really viable as a real solution, but the hack proves it can be done.
You can view different parts of a worksheet at the same time by either freezing panes, or indeed you can open multiple instances of the same document and view different areas within different instances.
