[A1].End(xlUp)
[A1].End(xlDown)
[A1].End(xlToLeft)
[A1].End(xlToRight)
is the VBA equivalent of being in Cell A1 and pressing Ctrl + Any arrow key. It will continue to travel in that direction until it hits the last cell of data, or if you use this command to move from a cell that is the last cell of data it will travel until it hits the next cell containing data.
If you wanted to find that last "used" cell in Column A, you could go to A65536 (for example, in an XL93-97 workbook) and press Ctrl + Up to "snap" to the last used cell. Or in VBA you would write:
Range("A65536").End(xlUp)
which again can be re-written as Range("A" & Rows.Count).End(xlUp)
for compatibility reasons across workbooks with different numbers of rows.