Lens
A lens is any medium that transmits light and has an Index of Refraction and shape designed to bend light. Lenses come in many forms but there's a few critical ones that relate to EM.
Crystalline Lens
This is the lens inside your eyeballs. It is not actually the strongest part of your internal refractive state but it is the part that is adjustable. The ciliary muscle can flex this lens and change it's power. The ability to change the power of this lens to focus on near work is called accommodation. As you age, the crystalline lens thickens and is prone to presbyopia and cataracts.
Glasses or Contacts
If the refractive state of your eye causes myopia or hyperopia then you probably wear contacts or glasses. The EM method addresses the spherical and cylinder components of your prescription. The strength of your lenses are measured in diopters.
Glasses
Glasses are lenses worn in front of the face, typically at a vertex distance of 10 to 13 cm. Glasses are easily changed and have lower risks than contact lenses, but at higher powers may have some tradeoffs.
Distortion Effects
Glasses are only perfectly tuned through their optical centers. If your Pupillary Distance, Pupillary height, or lens adjustment are off, you might not be looking through the optical center of the lens, and you are certainly not looking through the optical center when you look around instead of straight ahead. The power of the lens will vary, and unintentional cylinder can be introduced. You may also not be wearing the lens at the intended vertex distance, affecting the effective power of the lens. The stronger the lens the more of a difference this makes.
Contacts
Contact lenses are in physical contact with your eye, and have a vertex distance of 0. They carry high risks of infection, dry eye and other eye injuries, and require extensive care, and are hard to swap out multiple times a day, but have fewer distortion effects than glasses at higher powers.