They are somewhat covered in the FAQ, linked to from the bottom of the page. Some excerpts:
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The Freelancer Plan is a free plan with the option to purchase $20 packs which allows you to rent any 5 articles for up to 1 year from the start of the plan. Once an article has been rented, it can be read as often as you like until its expiration in 30 days. Any pre-paid rentals that remain unused after 1 year will become void.
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The Professional Plan is $40 per month and entitles you to "virtually" unlimited rentals each month, i.e. you can rent up to 40 articles per month and/or purchase and download up to 40 PDFs per month for a 20% discount. If you need even more access, please contact us because you are extraordinary!
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5-minute rentals are a great way to scan through any rentable article, for free. You get to read the complete full-text of an article in your browser for 5 minutes.
You can do as many 5-minute rentals as you want for free-- just not the same article twice in the same day. Rental tokens are available for unlimited rental access to the articles for 30 days.
Copying, printing, and downloading aren't included with 5-minute rentals.