Both words are nouns. "Effect" means the end result of a cause. For example, there is a cause-and-effect relationship when a moving object hits a stationary object (the energy is transferred). Or when you water a plant so that it grows, the "water" is the cause and the plant growing is the "effect." Efficiency is defined well above by Eliot. Efficiency is a word to describe how well something operates. If you can get showered, dressed and ready in the morning in 15 minutes but it takes me 45 minutes to do the same thing because I move slower, you have greater efficiency. HOWEVER, if it takes you 15 minutes to get ready in the morning and it takes me 45 minutes to get ready in the morning, but I shave and put on cologne and iron my clothes but you don't shave and don't wear perfume and wear wrinkled clothes, you don't necessarily have higher efficiency because we can't compare ourselves directly to each other. A car has a higher efficiency (is more efficient) than a bike in terms of traveling because you use less energy to press on a gas pedal in a car and can travel further and faster than you can if you're on a bike. You can't say a car has better gas efficiency than a bike, though, because a bike doesn't use gas, so they can't be compared in that way.

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effect = 效应 efficiency = 效率 Efficiency compares two things. For example, How many kilometers do you get from 1 litre of petrol in a car. It can also measure effectiveness against power consumption. How effective is the machine for each kilowatt of power used. Thus the confusion. In fact, they are two very different words.