Scroll down to read about 'Hanuman Chalisa' in English.
Hanuman Chalisa : History
हनुमान चालीसा : इतिहास
This hymn (स्तोत्र, स्तुतिगीत) was written by Gosvāmī Ŧulasīđās (गोस्वामी तुलसीदास) in the sixteenth century. Hanumān Chālīsā is written in Avađhī (अवधी; Awadhi). Avađhī language can be thought of as a root of Hinđī.
Ŧulasīđās is called Mahākavi (महाकवि) which means a great poet. This hymn/poem is simple and utmost rhythmical. Even if you don't understand Hinđī, you may enjoy hearing it.
Ŧulasīđās wrote these forty verses in the praise of Lord Hanumān (हनुमान). Forty in Hinđī is called 'chālīs' (चालीस; chaalees) and the hymn containing forty verses is called 'chālīsā' (चालीसा). These verses are called chaupāEE (चौपाई) which means four line stanzas.
Since he wrote this hymn others have written Chālīsā/s for other gods and godesses!
Lord Hanumān was beloved of Lord Rām (मर्यादा पुरुषोत्तम श्री·राम·चन्द्र; Mar‘yādā Puruṣhoŧŧam Shrī·Rām·chanđra). Another (bigger) work of Ŧulasīđās is the well known Epic called 'Rāmchariŧamānas' (रामचरितमानस).
Millions of Hinđū/s recite Hanumān Chālīsā on every Tuesday and Saturday. A lot of them believe that chanting this hymn will solve their problems. They believe Lord Hanumān is mahābalī (very strong and powerful). They lovingly call Him Bajrang·balī (बजरंगबली). Bajrang means someone whose body (अंग; an'ga) is like bajra (बज्र). Bajra is a dialect of vajra (वज्र) which means very hard like iron/spear. Often his statues are painted in saffron (bright red) or sinđūr (सिंदूर). [see footnote]
Generally Hanumān-jī is visualised as a bānar (बानर) which is a dialect of 'vānar' (वानर). Many Hinđū/s interpret the word 'bānar' as monkey-man. But some people say that the word _vānar means a man (नर; nar) who lives in forest (वन; van).
True worship of Bajrang·balī is to try to become strong (if your health and age permits) and help weaker Hinđūs.
Note : Lots of married Hinđū women apply a bit of sinđūr. This sinđūr should be based on turmeric and lime. But greedy manufacturers use mercury sulfide and red lead, which are toxic.