When choosing a mobile phone there are a number of different designs which offer variable user experiences. The one you choose may be decided by specific reasons or a good experience with a particular handset in the past.
The "standard" form factor, used on the very first mobile phones and still popular today. With this type of phone there is no protection for the screen or keyboard. Be careful not to keep it in a pocket with keys or other items that could scratch or damage the screen. Also make sure that the keyboard is locked as accidentally dialling someone or accessing the internet (many browser keys seem to be placed deliberately in a position where accidental use is encouraged!) could incur extra charges.
Usually the top part of the phone containing the screen slides over the keyboard. This means that the keyboard is protected but the screen remains exposed. They can be prone to screen damage due to broken connectors in the slide mechanism.
Very rare nowadays to see a flip phone. The advantages are both the keyboard and main screen are protected. Outer screens remain at risk of scratching and breakage. When opened the phones should give a decent call experience with earpiece and microphone naturally in the right place.
More and more modern mobile phones are touchscreen. They can be used by touching with your fingers or a stylus. Keyboard use is via a "virtual keyboard" that appears on the main screen. Different manufacturers utilise technologies that give a different feel to their touchscreens. Some users swear by them but others cannot get on with them. It is easier to make a mistake than with a traditional keyboard.
Some phones are now being released that feature more than one of these technologies. For instance they have a touchscreen top panel which slides out to reveal a standard keyboard. The best of both worlds or just more things to go wrong??