Ronen,
Numbers is constructed as follows:
Document: A single file containing one or more sheets. Documents have no provision for sharing data with other documents.
Sheet: A Sheet is the canvas on which the contents are placed. Sheets may hold one or more Tables, Shapes, Text Boxes, and Graphics.
Table: A Table in Numbers is a bounded array of cells which may be programmed to perform functions on data found in other cells, whether the other cells are in the same table or not, and without regard to what Sheet the referenced table is on. In the sense that there may be numerous tables per sheet, a Numbers Table is somewhat analogous to a Named Range in Excel. External references to cells must contain the External Table's name, and if that name isn't unique in the Document, the reference must also include the Sheet's Name.
Sheets are just a way of organizing the content. Each may be displayed as a canvas without boundaries, or they may be organized in pages in a What You See Is What You Get format. When you Print, you may Print your entire document, or a particular sheet, or a particular page.
Each Sheet may be individually formatted as either Landscape or Portrait, and all pages in a given Sheet must have the same orientation.
Does that touch on any of your questions?
Jerry