The fill handle is the small square at the bottom-right corner of the selection. Drag it down or across to copy a formula into the cells you drag over. Double-click it to fill down as far as the column next to it has data, which saves a lot of dragging on long lists.
When you copy a formula, its references move with it. =C2*D2 in E2 becomes =C3*D3 in E3 and =C4*D4 in E4. These are called relative references: they mean "the cell two to my left" rather than "C2".
That's usually what you want, because each row works on its own numbers. It also makes a running total easy: =C2+B3 copied down keeps adding the next value to the total above it.
The keyboard way: select the formula cell and the cells below it, then press Ctrl+D (Mac: Cmd+D) to fill down.