There are multiple types of Goals / Metrics in the Effective Experiments goal manager.

Visits / Conversions - If a goal is measured when a user lands on a page or triggers a a conversion event. If the number of unique goal conversions will always be less than the number of unique visitors, select this goal. When a test is complete, you will be asked to input the number of unique conversions for this type of goal.
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Define your visits/conversion goal in a descriptive manner
"Completed checkout" 

is better than "5% checkout completions" which is better placed in your hypothesis.

Events - If a goal is measured based on actions (eg. video views, pages viewed) and the number of actions per user can be greater than the number of unique visitors, select events. 

Revenue - If you want to input the monetary values for each variation, select this goal. It doesn't have any input as revenue is fixed. During the results stage, you will input revenue numbers per variation and it will work out revenue per visitor. Make sure you define your currency in the project settings 

Revenue per day - if you want to add an average revenue per day figure instead, then add this as a goal and the system will work out the total revenue