As an example, buying toner for the office printer is an expense. However, the printer itself is generally not an expense, but an asset purchase. (In some larger companies, purchases of assets under a certain amount are treated as expenses due to the small amount involved.)
On a financial statement, a "prepaid expense" is an amount which is for an expense covering a period of time in the future. A common example is an insurance premium, which often covers portions of more than one fiscal year.