An anchor text is the clickable text -most often underlined and blue- in a hyperlink.
Anchor texts associated with backlinks and internal links are key factors for SEO. Search engine algorithms and evidently Google’s algorithm give more weight to descriptive anchor texts. For search engines, the descriptive anchor text is a relevancy signal when it matches the query.
For SEO purposes, professionnals are searching to multiply descriptive and relevant anchor texts for targeted queries. Many "legitimate" or SEO black hat practises are used for that. For instance, a good practise is to use an HTML sitemap with legitimate anchor texts.
Black hat practises for "good" backlink anchors are :
press releases on directories
Since 2012, Google has used his new filter algorithm Penguin to detect and penalyze artificial backlinks and anchor text over optimization. A bad backlink profile with too many similar anchor texts can lead to sanctions.