Contact lenses are sometimes also referred to as prescription lenses. This is because they can only be obtained once you have been given a prescription from an eye care professional. Contact lenses are classified as medical devices, since wearing the wrong lens can do more harm than good, and therefore suppliers are not allowed to sell lenses to a person who doesn't have a valid prescription.
Eyeglass lenses sometimes also fall under the category of prescription lenses, as they are most commonly worn by people who have been given a prescription for vision correction. If you wear both glasses and contacts, learn to distinguish which prescription is for which vision correction solution, as the measurements are often very different. An eyeglass prescription will contain details of the sphere, cylinder and axis of the lens - something that is also specified for toric lenses - while your contact lens prescription also states the base curve and diameter, as well as brand name, material and water content.
Whatever your contact lens needs, don't buy contact lenses without seeing your optician first, and always stick to their advice once you have received your prescription. There are lenses available for nearsightedness, farsightedness, astigmatism and presbyopia, some of which are more complex to fit and hence may require more regular check-ups.
It is up to you where you want to buy your contact lenses, but either way your optician is obliged to give you a copy of your prescription after your check-up. If they seem reluctant to give you the details you need, quote the Opticians Act. The contact lens prescription legally belongs to you, and it shouldn't cost anything at all if you have paid for an examination.
It does occasionally happen that contact lens wearers come across so called non-prescription lenses, but these are no longer legal in the UK and could put your eye sight at risk. If you are encouraged to buy a different brand to that on your prescription, or if a supplier doesn't ask for your optician's details, you have a reason to be suspicious. Contact lenses are entirely safe when prescribed by an eye care professional - but there is a reason that you need a prescription.
In addition to normal spherical lenses, coloured contacts are also regarded as prescription lenses and must be fitted by a professional even if you only wear them for aesthetic reasons. You may not need powers and cylinders, but you want to make sure that the material you choose is suitable to your eye and tears.
Acuvue (Johnson & Johnson), Ciba Vision, CooperVision, and Bausch & Lomb are the most well-known manufacturers of prescription lenses. Your optician will recommend a brand that suits your eye, and you should stick to that even if other unknown brands are available for a lot less. Most likely, your high-quality brand contact lenses are also available for a discounted price online - and with these, you don't have to compromise on your safety.