Manual Juicers are operated by hand and have no electric or battery mechanism attached. It is simply a basic tool to use to “squeeze” the juice out of the fruit.
Manual juicers work best for citrus and very “juicy” fruits that are easy to extract. It does not extract a high percentage of juice (as do the other types of juicers), and the produce you'll able to use with it is limited to basically only citrus.
They are inexpensive and good for a quick juice of lime (for making guacamole or in a glass of water, for example).
They clean up very quickly and easily. The hand manual juicer is a great tool to have in hand for your kitchen for small juicing needs. However, I do not think it does an adequate job of taking the place of a juicing machine as its juice yield, nutritious output and ease of use is almost incomparable. There are also many manual wheatgrass juicers that fall into this category of juicers.
A note about manual wheatgrass juicers:
There are several models and variations on the market, including specialized “crank” type for wheatgrass, named Z-Star Wheatgrass juicers that get high ratings. This juicer, however, costs approximately $100. In my opinion it makes more sense to spend the money on a more versatile juicer unless wheatgrass is the only thing you'd like to juice (and there are some cases where this would make good sense).
There are also larger orange juice machines that are electric or hand crank, that are only designed to juice citrus. Overall they work well if that is all you are interested in juicing.
However, if you are interested in juicing other types of produce, these machines will not be able to do that. Some people enjoy just orange juice for breakfast on a weekend morning for the family. So if that's you, these larger citrus machines are for you.
And as far as some of the citrus hand crank juicers go, many are attractive to leave on your countertop.