The main difference is the amount paid out. Level term life insurance provides a fixed pay-out if you die within the term of the policy. With decreasing term life insurance, the pay-out decreases in value over the term of the policy.
For example, a level term policy taken out for 25 years for cover up to £100,000 will pay out that sum, whether the policyholder dies within three years or 20 years. A decreasing term policy decreases in value over the term of the policy. The longer the policy lasts, the less of that £100,000 amount will be paid out if you die.