Tengwar is the name of Tolkien’s invented writing system used for many Elvish texts.
Tolkien designed the Tengwar as a featural script, meaning that the shapes of its letters reflect aspects of the sounds they represent. In his mythology, the Elf Fëanor created the script, and it was later adapted for several languages of Middle-earth.
Tengwar is a writing system, not a language. It can represent languages such as Quenya and Sindarin, and it can also be used for other tongues. This distinction is important because readers often confuse the script with one of Tolkien’s invented languages.
The inscription on the One Ring is commonly shown in Tengwar characters. However, the language of that inscription is the Black Speech of Mordor, not an Elvish language. Tolkien’s calligraphic design and linguistic detail helped make the script recognizable beyond the books.