With an Unsecured Loan, the borrower undertakes to make the repayments, and the lender will make a judgement on whether or not to lend based on their creditworthiness. The risks to the lender that they will not be repaid are relatively high, which means that the interest charged will be higher than a secured loan or the amount lent, lower.
A Secured Loan is when a borrower put 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. When you take out a loan to buy a factory or house, the property itself is securing the loan.
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.