At first glance, one can distinguish clearly the West Semitic origin of this writing system. For instance, the symbol for a resembles greatly Semitic letter 'alif. Similarly, dha, tha, la, and ra all appear quite close to their Semitic counterparts/ancestors. There is, also, a slightly different school of thought that proposes a Southern Semitic origin. Still, a third school of thought holds that the Brahmi script came from the Indus Valley Script. However, the lack of any textual evidence between the end of the Harappan period at around 1900 BC and the first Brahmi and Kharosthi inscriptions at roughtly 500 BC makes the Indus origin of Brahmi highly unlikely. More research (as in digs) should be conducted, though, to either prove or disprove this theory.
Brahmi is a syllabary, it consists of syllables only, if we state that single vowels are also syllables. Each character carries a consonant followed by the vowel "a", much like Old Persian or Meroïtic. However, unlike these two systems, Brahmi indicates the same consonant with a different vowel with extra strokes attached to the character. Brahmi is written from the left to the right.
Already in the last centuries BC the script was divided into 3 varieties: northern, south-eastern, and southern. Dialectal differences consisted of the shape of the symbols, though the system remained the same. First separate branches emerged in the 5th century AD. The Brahmi script is the ancestor of practically all modern Indian writing systems, at all there are about 40 varieties of them nowadays, including Tibetan, Singhalese, Sharada, Newari, Bengali, Oriya, Gujarati, Gurmukhi, Lahnda, Kannada, Telugu, Tamil, Malayalam, Burmese, Khmer, Lao, Thai, Devanagari. In addition, many other Asian scripts, even Japanese to a very small extent (vowel order), were also derived from Indian script. Thus the Brahmi script was the Indian equivalent of the Greek script that gave arise to a host of different systems.
Languages which used Brahmi as their script: Indo-Aryan
(Vedic, Sanskrit, Prakrits, Pali), Iranian