Lip Fillers are dermal fillers which are made from naturally occurring hyaluronic acid. Every lip is different. Lip fillers can enhance the shape of the natural lip and add volume but it's not possible to change the shape of the lip completely. A set of lips on one person wont look right on another person because it's important to maintain natural beauty ratios with the rest of the face.
There are many brands of fillers, some of the ones you may recognise are: Juvederm, Restylane, Teosyal, and Belotero. Each brand has a variety of fillers which vary in the results they will produce, for example some are much softer than others.
Lip fillers are also injected using different injection techniques, this impacts the end of result and whether it looks natural or not.
Lip fillers are temporary as they are naturally absorbed by the body. Lip fillers tend to last 4-6 months on average.
Lip fillers are safe but do carry some risks, albeit small risks and it is therefore imperative to have a consultation before treatment with a qualified medical professional who can deal with any complications should they arise.