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A magnifier (or magnifying glass) and loupe all follow some basic rules of optics. When shopping for a magnifying glass or loupe, keep these rules in mind to find the best magnifying device for your needs. While a loupe is technically a magnifier, the difference between the two is simple: a loupe is held closer to the eye and offers higher magnification ranges while a magnifying glass is held further from the eye. 
As the magnification of a magnifier increases, lens size goes down. For example, a 2in x 4in rectangular 2x magnifier has a lens size of 4 inches. This is suitable for reading a book or text or just taking a closer look at things in general. On the other hand, a 10x magnifier usually has a lens size of about a 1/2 inch or less and will therefore offer a much smaller "window" to see things. It is great for high magnification and a super close look at many objects, but it will not perform as well if reading a book. Unfortunately, you can't have it both ways, so you need to decide if magnification or lens size is more important for your application.
Focal Distance of Magnifiers
As magnification goes up, focal distance goes down. Think of focal distance as the distance the magnifier needs to be from the subject to be in focus. A 10x magnifier, such as a jewlers loupe, will have a very short focal distance, usually about one inch or less. In other words, the magnifier must be held one inch from the subject to properly focus and at such a short focal distance, there will not be much room for error. A fraction of an inch too close or too far will cause the magnifier to go out of focus.
On the other hand, a large 2x magnifier will have a much greater focal distance, often as much as 5 inches, and it will be much more forgiving as to the exact distance. You can be off a couple inches either side of the listed focal length and still be in focus, though you may not get maximum magnification or clarity.
Magnifiers: Unique Optical Instruments
By the way, magnifiers are not designed to be used to see distant objects. If you are trying to see objects more than a foot or two away, you need to look at a monocular or binocular, some of which can also focus within two feet.
Magnifiers are unique optical instruments in that the magnification (how many times the image is enlarged over normal) can change as you hold the magnifier closer or farther from the subject, though you are much more likely to notice this phenomenon with low power magnifiers. Since magnification can change with the distance the magnifier is held from the object, the lens in a magnifier is sometimes given a diopter rating. Diopter refers to the strength of the lens, not the magnification, and it rates all lenses as if they had the same focal length. However, there is no need to worry about diopter unless you have a specific technical need to do so. Just check the stated magnification and focal distance if listed and go from there.
Text Magnifiers
Text magnifiers are used to enlarge text. Some are page size but very low power (2x) because of their size. However, most magnifiers are much smaller, covering 4 lines of text at a time. All are designed to be laid flat on a page. We even offer magnifying glasses with lights to help you read in dim environments.
Hands-Free Magnifiers
Hands-free magnifiers are widely used by hobbyists to work on projects that require the use of both hands. They can be as simple as a magnifier on a stand, or they can be visor mounted units such as dental loupes.
Wallet and billfold magnifiers are the size of a credit card and some are even flexible enough to not crack if accidentally sat on.
Loupe
A loupe is a magnifier designed to be held close to the eye, though the term is often used for magnifiers in general. Some loupes can be attached to glasses like the image above. You can hold a loupe at arms length over a subject and it will still work, but the field of view (amount of the subject you can see) will be very narrow. If you want the entire field of view, hold the loupe and the subject up close to your eye. The older traditional loupe is placed and held with the socket of your eye as in an old fashioned monocle, but these days, most loupes are handheld and typically have an attached folding case which doubles as a handle.
Why Use a Loupe?
Loupes have many uses. Jewelers loupes offer the highest optical correction since subtle differences in gem color greatly influence gem value. Film loupes are also very high quality and are used to evaluate photo negatives and slides. Dentist loupes are used during examinations and procedures. Botanists and naturalists also use loupes to study flower and insects and a wide range of natural wonders. Measuring loupes have scales of varying types mounted in the magnifier to measure very small items. Since most loupes are fairly high magnification, they are also quite small and fit easily in a pocket.
