Devenagari is the prime script of India used for its official Hindi language. India has many other scripts but they are all similar to Devanagari in nature and history.
Devanagari characters are quite elaborate, all connected by a line at the top. Devanagari is the native script for ~500 million people (7,7% world population) and is used in 1% world land area.
Nearly every Devanagari letter means a simple syllable of some specific consonant and the “inherent vowel” A.
That inherent vowel may be changed into another vowel if by diacritical marks. These diacritical marks range from small dots above/below to letter-sized symbols to the side.
A letter with diacritical marks thus usually means any syllable of a single consonant and a single non-A vowel where the letter shows the consonant and the mark shows the vowel.
Additionally, some letters without diacritical marks mean vowels without consonants. Consonants without vowels may be written by combining a letter with a “no vowel” diacritical mark. A syllable of multiple consonants and single vowel may be written by a ligature – an atypical combination of several letters representing those consonants and a diacritical mark representing vowel.
History of Devanagari
Devanagari is one of many similar Indian scripts known as abugidas. They all have characters with same meanings (but different appearance) and all could be used to write Sanskrit (the language of Hindu sacred texts).
Devanagari is one of the northern Indian scripts known for their less rounded appearance. Northern and southern Indian scripts separated at ~3rd century AD. ~8th century Nagari script evolved. Nagari gave birth to Devanagari (used for Hindi) and Bengal script in 11th century AD.
Colonial period (British rule) led to introduction of spaces and Latin marks (not previously used) at ~16th century.
In 1947 India became independent. Hindi is its official language and thus Devanagari arguably the main script. Some Indian languages (such as Marathi) then adopted Devanagari to replace their original writing systems. A widespread believe of southern Asia that every language must have its own script however ensures that India remains a country with many scripts and Devanagari is used nativelly by less than 50% Indians.