The more calls you make, websites you visit and videos you play will mean you will need to charge your battery more often. Some phones have a larger battery and can last more than twice as long but this can mean your phone is bigger or heavier.
Your smartphone will need to work much harder if you are playing games with lots of fast movement so if your budget can stretch look for a model with fast graphics speed.
When using your smartphone some things will be more demanding on the technology inside than others. If you are likely to do multiple things at once, stream lots of movies or play fast moving games then a smartphones with higher performance will really make a difference.
Screen quality can make a real difference if you browse quality photos, watch high definition movies or read a lot. Lower quality models can be duller and slightly blurry up to the higher end models where the richness of colour and sharpness of text really stand out.
Some smartphones are more protected than others. We rate the protection from water (and dust) based on how well they cope in certain situations.
Making a call whilst walking through a busy city can make for uncomfortable listening on the other end. Smartphones that include noise cancelling technology cut out background noises so you sound much clearer. Most smartphones include this these days but there are a few that don't so if you make a lot of calls on the move look out for this.
The front camera above the screen is mostly used for selfies and video calls. Typically these have been lower quality than the rear camera but more recent models have seen the quality improve significantly.
Like most electronic devices, smartphones are being upgraded regularly. Latest models tend to have better features but you can often find that an older model will do everything you need and has been discounted giving good value for money.
The operating system is what you will spend most time using - it controls everything you do on the screen and in apps. There are really only 2 to consider and its mostly down to your personal preference - Apple's iOS has a clean consistent layout with lots of high quality apps. Google's Android is available on a wide range of devices with a wide selection of apps.
If you only use a few apps and take occasional photos then the smaller sizes should be ok but consider larger if you play games and tend to snap everything that moves. You can manage with less if you use 'cloud storage' which lets you keep your photos and files online but be wary this may incur monthly charges.
If you are like many people, the rear camera in your smartphone will be the only camera you need. Whilst high end models can be expensive, the convenience of not having to carry a separate camera may be worth it.
Whilst all smartphones come with WiFI built in, if you are streaming movies it is worth considering smartphones that can cope with a faster connection.
A pen lets you write and draw on the screen as well as using your finger. If you need to be able to take handwritten notes or make quick sketches the technology is now very close to using a traditional pen. There are only a few models available with a pen and you will need to be sure you will use this regularly for it to be worth the extra expense.
Using lots of apps or taking lots of photos can soon eat up your storage space. Smartphones with expansion can plug in memory cards giving you more storage and an easy way to transfer your photos and files. If you aren't sure how much storage you will need, a model with Expansion can give you the peace of mind. Note that Apple iPhones do not include expansion.
The quality of smartphone video can be stunning - a dedicated camcorder still has the edge when zooming - but the convenience of your smartphone always in your pocket often outweighs this. Most models are ok for general playback but if plan to play on a TV you need to look at better quality models.
If you need to give your battery a quick boost without having to wait too long, smartphones with fast charge are worth considering. You will need to ensure you have a compatible charger and cable to get the benefit. Many of the latest models include this and some can fully charge up to 4x faster and give a useful 30% charge in less than 30 minutes.
The dimensions of the device takes into account its height, width and depth. Some models can have a similar screen size but be bulkier. The latest models tend to be slimmer and have less edge around the screen.
Think about how you will use it - a large screen will give you comfortable browsing and a better movie-watching experience. If you are on the move a smaller screen will be more portable and easier to carry around in your bag.
Compared to other devices like tablets and laptops - most smartphones are light enough to be carried around all day so this probably isn't as big a factor as its size.