What do the components of a base editor do?

The basic components of a base editor are: Cas9 Nickase, nucleoside deaminase, and sometimes a DNA glycosylase inhibitor. 

1. Cas9 nickase (nCas9) is a modified version of the Cas9 enzyme with mutations in one of its key amino acid residues responsible for DNA cleavage. This alteration allows nCas9 to still bind to a guide RNA (gRNA) and locate the DNA sequence complementary to the gRNA spacer. However, nCas9 can only create a nick in one strand of the DNA, rather than a full double-strand break.
2. Nucleoside deaminase, an enzyme found in base editors, alters nucleosides by eliminating an amino group. When combined with the nickase enzyme, either an adenosine deaminase (such as engineered TadA) or cytosine deaminase (such as APOBEC) determines whether the resulting base editor is is an adenine base editor (ABE) or a cytosine base editor (CBE). 
3. A DNA glycosylase inhibitor, such as UGI (uracil DNA glycosylase inhibitor), prevents the action of DNA glycosylase enzymes. Attaching an UGI inhibitor to dCas9 stops the repair process that would otherwise revert the edited uracil back to its original cytosine form.