In one sentence, describe what the following article is about:

Most carriers will offer a range of phone options across operating systems (no OS is specific to a carrier). Major carriers often subsidize phones or offer different payment plans and contract combinations to reduce the up-front cost of smartphones. Some carriers, like T-Mobile, allow you to forego a contract while paying off the phone as part of your monthly costs. Cancelling your service early will force you to pay the remainder of the phone's costs at once. Unlocked phones are phones purchased outside of a carrier and thus not linked with a phone service contract. They are more expensive but allow you much more flexibility if you ever need to switch phone carriers. If buying an unlocked phone, make sure to doublecheck that the model is compatible with your specific carrier's network. Most carriers have a webpage where you can check compatibility with your phone model's ID information, (for instance Verizon, or AT&T). Phone service carriers will usually offer a wide range of prepaid monthly plan options for phone minutes, texts, and data over the cellular network. You may be able to cut monthly costs by not purchasing a data plan at all, but this means you won't be able to access the internet from your phone if not on wifi. Screen size is measured corner to corner diagonally. Ultimately, screen size is a matter of preference. Smaller screen phones may fit better in your pocket and are often cheaper. Larger displays may be preferable if you plan to watch a lot of videos. iPhone offers the "SE" series for compact phones and the "Plus" series for an extra large screen. Android phones come in a wide range of sizes: there are smaller budget models such as Moto G or Galaxy S Mini, higher end models like the Galaxy S or the HTC One series, and oversized models like Galaxy Note or Nexus 6P. Newer phones are typically faster and more powerful than their respective older versions but will come at a higher cost. In particular, older model phones will have a more difficult time running modern applications. For the budget-conscious, a good compromise is to wait for a new model of your desired smartphone to become available and to then take advantage of a drop in price of the other models. When a new phone model premieres interest in older models will immediately go down and the price will often change to reflect that. Regardless of your choice, understand that technology moves very quickly and that newer phone models will continue to appear. Eventually every smartphone will seem old or obsolete. A phone's storage (usually listed in gigabytes or GB) is a measure of how many files (photos, videos, apps) it can store at any one time. Storage space strongly affects the price of the smartphone so consider how much you're likely to need before settling on a phone model. For example, storage space is the only difference between a 16GB iPhone 6 and 32GB iPhone 6, 16GB is estimated to hold about 10,000 pictures or 4000 songs -- but keep in mind that your phone storage must also accommodate all your downloaded apps. Some Android phones (but not all) support storage expansion with the purchase of a microSD card. iPhones do not support storage expansion after purchase. Although smartphones are known for taking high-quality photos in general, actual picture quality will vary quite a bit between brands and models. The best way to measure camera quality in a phone is to search for sample pictures online taken with that smartphone model or to demo the camera yourself. While manufacturers often advertise a camera's megapixel count, features like ISO, low-light performance, brightness, and noise reduction are equally if not more important to consider. Most modern smartphones come equipped with front and rear-facing cameras and flash, and will support third party add-ons (such as lens attachments). iPhones are well known for their high quality camera hardware/software. Battery technology is improving steadily so newer phones tend to have longer battery life, however your usage habits are what really dictate how long the battery lasts. Talking on the phone, gaming, and using phones outside of wifi range will all drain a battery more quickly. Average smartphone battery life can range anywhere from 8-18 hours. Most flagship Android models will not support replaceable batteries. iPhones do not support replaceable batteries on any model. Some newer Android phones utilize a quick charge technology to help recharge their large batteries faster (e.g. Samsung Galaxy S series or Motorola Droid Turbo series). Manufacturers claim phones with quick charge can achieve 50% charge in about 30 minutes.

Summary:
Pick a carrier. Pick a phone service and data plan that works for you. Pick a screen size. Decide how new you want your phone model to be. Check the storage space. Consider camera quality. Consider phone battery life.