In 2015, according to CNet, 47% of smartphones sold in the United States were purchased through monthly installment plans.
Monthly installment plans allow you to purchase a cell phone over the course upto 30 months
Its basically a finance agreement, like paying for a car - instead of paying out the full price right at the start, you can spread the cost over a longer period of time. This is great if you don't have the $700+ to buy a new iPhone or Samsung - monthly payment plans typically cost between $20-$30 per month, depending on the price of the phone itself.
Each carrier offers its own monthly installment plans making payment prices for each carrier different.
Remember - you will need to pass a credit check with the carrier
There are a couple of different options available to you:
Lease - this is a monthly payment plan, at the end of the lease period you don't actually own the phone outright. Basically you are renting the phone. At the end of the term you can either return it to the carrier and choose a new phone, or pay out the remaining balance to buy the phone.
Finance - also a type of monthly installment plan, the key difference here is that you actually own the phone at the end of the term. Finance plans will typically be slightly more per month than the lease plan - because you are paying off the full amount.
Early Upgrade - these are monthly plans where you have the option to upgrade to a new phone for no extra cost during the payment term. Remember though that this will extend your agreement and the monthly cost will be higher than that of a normal lease or finance plan
Monthly installment plans give you the option to not spend a large amount of money upfront and instead pay a minimum price over time. I have provided three links below to show you some monthly installment plan options. The first link is for an iPhone 6s 16 GB, the second link is for an iPhone 5s 16 GB and the third link is for a Samsung Galaxy s6 32 GB
Remember the only way to get out of a monthly installment plan is to pay the remaining balance on your contract or complete all payments for the full term of the agreement