word usage - What is the difference between effect and affect?

I am confused about these two words how they differ in usage. Would be easy if explained with example.

Answer

There are lots of examples of usage for each sense of affect (OALD definition) and effect (OALD definition) at their dictionary definitions.

However, an interesting case appears with the following sentences.

The UN affects changes in our foreign policy.

The UN effects changes in our foreign policy.

Both sentences are valid, but have a different meaning. In the first, the meaning is that the UN influences planned changes in our foreign policy. In the second, the UN initiates changes in our foreign policy.