Leeches
Leeches are cards that you keep on forgetting. Because they require so many reviews, they take up a lot more of your time than other cards.
In order to save yourself time and frustration, it's important to identify leeches. By default, leeches are cards that have over 15 failures. When Anki detects a leech, it will suspend the card, tag it with 'Leech', and show a message that it has been suspended. You can change the threshold for identifying a leech, and whether to suspend or not, in Settings>Deck Properties>Advanced.
Anki will continue to issue leech warnings periodically for a difficult card. The warning interval is half the initial leech threshold. That is, if you have Anki configured to warn at 16 failures, future warnings will happen every 8 failures (24, 32, 40, etc).
Once a leech is found, there are a number of ways you can handle them.
Leaving Leeches Suspended
Some leeches are caused by interference. For example, an English learner may have recently learnt the words "disappoint" and "disappear". As they look similar, the learner may find themselves confusing the two when trying to answer. In these situations, it's often productive to concentrate on just one idea. When that idea is firmly ingrained in your mind, you can then return to learning the other idea. So in these situations, you may want to unsuspend the leech a few months in the future.
Deleting Leeches
Another way to manage leeches is to delete them. Consider if the material you're struggling with is important enough to make it worth your while. By selectively deleting difficult and obscure items, you can dedicate more time to learning other material.
Editing Leeches
Another approach is to change the way the information is presented. Perhaps the cards you have created have too much information on them, or perhaps you're trying to memorize something without fully understanding it. Sometimes spending some time changing the way the fact is phrased can help. It's also a good time to think about making a mnemonic to help you remember.
Leeches on other versions of Anki
AnkiOnline and AnkiMini will automatically identify (and maybe suspend) leeches based on your desktop settings. They will not give you any indication this has happened, however, so it's worth checking cards with a leech tag in the desktop client occasionally.
More Information
SuperMemo also has an article on leeches: