Delete means just that, delete. If, for example, you high-light an entire row of data then right click and select delete, you'll notice that the entire row disappears and all the subsequent rows move up one spot. It is gone forever.
If you select "Clear" instead, all the data (including all formulae, values, formats etc.) are cleared from that row but the row remains as a blank row.
If you select "ClearContents", then only the cell values and formulae are cleared. Formats will remain and the row will remain in place.
I hope that this helps,