With an Unsecured Loan, the borrower undertakes to make the repayments, and the lender will make a judgement on whether or not to lend solely based on their creditworthiness.
A Secured Loan is when a borrower puts up a security(collateral) using something of value, such as a house, that the lender can take and sell to recover their losses if the borrower does not keep up with the loan repayments. Such loans include secured overdrafts and property gear-up and they are usually cheaper than unsecured loans or the quantum larger, as the risks for the lenders are lower.
Some lenders consider loan types like invoice financing and contract financing as secured loans too, especially if a strong paymaster discloses them. You can find more details on them on the same page.
When you take out a loan to buy an asset or house, the asset or property itself is securing the loan if the lender can repossess it directly such as by having a charge or lien over it and it would be considered a secured loan too.
In the UK, a secured loan is also called a debenture, whereas in the US, a long-term loan even when unsecured, is called a Debenture.