What is a guide molecule?

The guide molecule is used to target CRISPR-associated Cas9 endonuclease to specific genomic loci. Typically, guide molecules possess an RNA backbone, but it is possible to have DNA nucleotide hybrids. RNA guide molecules are most commonly designated as sgRNA (single guide-RNA), which contains a variable region of ~20 nucleotides that are specific for the targeted gene of interest (crRNA). Additionally, there is also a conserved scaffold sequence which is identical for Cas9 recruitment (tracrRNA).¹

Reference:
1. K. Chylinkski, et al., "The tracrRNA and Cas9 families of type II CRISPR-Cas immunity systems," RNA Biol 10(5):726-737, 2013.