Ketiv and qere
The written consonants (ketiv) and the reading tradition (qere) sometimes disagree. Both are in the Masoretic notes.
What you see in a printed Bible
BHS puts the ketiv in the text line and the qere in the margin (often with a small circle). You chant the qere. A teaching ḥumash may already print the reading form.
The Name
The standing example is יְהוָה: written as the four letters, read as אֲדֹנָי (or as אֱלֹהִים when already next to Adonai). The printed vowels on YHWH belong to the substitute, mixed onto the ketiv.
written Name; read Adonai
the usual qere of the Name
Ordinary words
Some ketiv/qere pairs are spelling variants, an unexpected gender, or a perpetual reading (for example the usual spoken form of Jerusalem). They are not free license to rewrite the verse. Check the masorah when a form looks "wrong."
Jerusalem (common printed qere shape)
Read next
Vowels and the Masoretes
The Masoretes added niqqud so a purely consonantal Bible could be read with a stable tradition.
Names of God
Several divine names and titles run through the Tanakh. Jewish reading treats the four-letter name with care.
Reading the Torah scroll
A sefer Torah is consonants only, in square script, on parchment, chanted with a memorized melody.