According to their website, Amazon sells cell phones in three categories: no-contract cell phones, unlocked phones and phones with plans. There is a wide variety of devices within each category.
Under the contract category, Amazon supports three major carriers: Sprint, AT&T and Verizon(wireless). Some of the phones offered here are the Samsung S4,S5 and S6 phones. The no-contract cell phones are those that come with prepaid service plans, which offer flexibility in terms of payment to a specific carrier and, according to Amazon, the bestseller for this category is the Nokia Lumia 635. The unlocked phones are phones which you can easily switch carrier sim cards as they are not locked to any specific carrier.