Basically leeches belong to the phylum annelida and comprise the subclass hirudinea. This particular subclass is because of a reason which we are going to see further in the post. These leeches prefer to live in moist places so most of them can be found in fresh water environments. Talking about their physical body, they do not have bristles like earthworms though both belong to the same phylum. Instead, they have two suckers, one at each end which help them to suck blood.
It is generally believed that leeches only feed on sucking blood, but it is not so. They also feed on other small invertebrates by swallowing them as a whole. Some leeches feed off decomposing bodies as they can not bite.
Leeches attach themselves to the host and release a combination of mucus and suction to remain attached. After this they release an anti-coagulant enzyme, called hirudine into the host's blood, which is probably why they belong to the subclass hirudinea. These leeches release an anesthetic into the body which is why we don't feel pain. But this presence of anesthetic haven't been proved yet, though scientists found some morphine like substance in them. This hirudine restricts the blood to clot thus allowing it to flow continuously till the host's body is able to clot it. Once they are done with sucking the blood, they just simply fall off the body to digest the blood they've sucked.
There are different views of how to remove a leech. Some say that it can be removed by introducing some kind of fire, or pouring chemicals like alcohol but in that case the leech may vomit the blood it has already sucked into the body again which may result in infections. So picking it with hand is advised by some other people. But frankly speaking, a leech sucking blood wouldn't really effect your blood level. And once it is done, it will anyway drop off.
Besides this, they are also used for medicinal treatments. In simple words, when a situation arises where the body fluids like blood, bile, phlegm are not in control, these leeches are used to balance them. They have applications in the plastic and reconstructive surgeries in good olden days. Hirudin is under research to be used as coagulant. Interestingly, a leech used for one person is never used to another to ensure that any virus doesn't transfer from the former.
Image courtesy - wikipedia
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