There are typically six different types of lenses available for eyeglasses. While their names and descriptions may vary depending on the store, they usually consist of: Basic (1.5), Thin (1.56), Thin & Lite (1.61), Hi Impact Resistant (1.6), Super Thin (1.67), and Ultra Thin (1.74). The number one thing to remember is this: the higher the index of the lens, the thinner the lens is.
Basic (1.5) are simple plastic lenses with the standard refractive index; they are recommended for very low prescriptions. Thinner than the Basic (1.5), the Thin (1.56) are a bit lighter and generally for slightly higher prescription needs. Thin & Lite (1.61) lenses are often times the most popular. They are still a very noticeably lighter, thinner lens, while being the least expensive upgrade. These lenses are great for rimless glasses. Polycarbonate or similar Hi Impact Resistant (1.6) lenses are a more resistant plastic than the conventional lens type. The quality of these lenses is usually a bit better, more durable - yet still lightweight as they are a higher index. Super Thin (1.67) lenses have a much lower distortion and are incredibly sharp. These lenses are mainly recommended for high prescriptions - for an SPH anywhere between -7.00 to +7.00. Ultra Thin (1.74) are the highest performing, thinnest lenses possible; they definitely rank above all others in quality. These lenses could be considered either an extravagant luxury - or a necessity, depending on your prescription needs.
Having glasses on your face is an annoyance - plain and simple. While there are ways to turn them into a great accessory, ultimately, you want this obligatory nuisance to simply be the most comfortable as possible. Therefore, whether or not you have very high prescription needs, upgrading your lenses to hold the next step up of thinner lenses is really helpful. Besides the understandable need to have a more aesthetically-pleasing look with thinner lenses, getting Thin & Lite (1.61) can literally take the weight off of you. The biggest problem with adding any kind of additional price to your eyeglasses is the fact that they can break the bank - with all the simplistic necessities. However, when you buy glasses online, you have so many more affordable options! Usually online eyeglasses are far less expensive than in-store, and without the commission-grabbing salesman breathing down your neck, you can order glasses in peace!