There are various ways of using the selection to ease the edition of the cells in the spreadsheet.
For instance, given a selection of cells (contiguous, or non-contiguous), data can be entered. The Enter key will accept the data and move the cursor to the next cell in the selection down the selection. The tab key will move the cursor to the rightmost cell (or it will scroll if you have reached the edge of the selection).
Control-Enter does the inverse of Enter (it moves to the previous cell in the selection, moving towards the top), while Ctrl-Tab moves the cursor to the left.
The active cell is indicated by a dark double line with a small square in the bottom right corner. The active cell is the the one that is currently in focus and will take any input for the keyboard.
To make a cell become the active selected cell, simply move the white-cross cursor over the cell and press the MB1.
A selection of multiple cells is indicated by a dark double line around the extents of the selection. The top-left cell appears much like a single selected cell while the rest of the selection is the inverse color.
To make a continuous selection of cells, click on the cell with the white_cross cursor and drag the extents of the selection until it comprises the area you wish to select, and release.
To select a set of cells that may not be continuous, use ctrl+MB1 and drag to argument an existing selection. The cells can be added individually or as parts of other selections.