The runes of the Elder Futhark were a writing system before they were anything else, and the surviving evidence for them is objects with letters on them.
Fourth in the row
Ansuz is the fourth character of the Elder Futhark, the twenty-four character row used in the earliest Germanic inscriptions from around the second century. It supplies the a of the sequence f, u, th, a, r, k from which the row takes its name.
A vowel that moved
This character's sound value did not stay still. As the language changed, the vowel it wrote changed with it, and the Younger Futhark that replaced the Elder row in Scandinavia inherited the character with a different value again. That instability is why transliterations of the same character vary between corpora, and it is a linguistic fact about the languages rather than a disagreement between scholars.
What survives
The Elder Futhark corpus consists largely of short inscriptions on brooches, weapon fittings, combs, bracteates and stones. Many are single names. Several of the longer texts have readings that remain contested, and the standard editions print alternatives rather than choosing.
The name
Ansuz is a reconstructed form, projected back from the names the medieval Old Norse and Old English rune poems give the characters of their own later rows. The word behind it means a god of a particular family of gods, and the Old English tradition, working in a Christian context, glosses its equivalent character differently.