Difference between APR and nominal rates

I know the APR is the better one to go by but can somebody actually explain the difference between the 2 ?

eg

BOI 2 yr fixed APR is 3.6% and nominal rate is 3.55%
EBS 2 yr fixed APR is 3.4% and nominal rate is 3.75%

Even though the APR is lower with BOI the monthly replayments are lower than the EBS

This crops up regularly and always eventually causes me to get confused again. Try these links:
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