Amazon today gave its Kindle users the ability to lend e-books for the first time, an innovation the Nook e-reader had first. The privilege comes with similar limits, however, as ReadWriteWeb explains. First of all, it's up to the publisher whether a book can be loaned, which could create a pretty random spread of loanable and non-loanable titles. Books can be shared only once, and for a period of 14 days.
During that time, you can't access the book yourself—a fair approximation of loaning a physical book—after which it reverts back to your control. And for now you can loan only via the Amazon website, not through the Kindle itself. (Go to the "Manage Your Kindle" section of your account.) The recipient has to accept the loan via email and download it, though they don't need a Kindle to read it. Various Kindle apps are available for them.