Suppose Alice wants to find Bob's public key. There are several possible ways. She could call him up and ask him to send her his public key via e-mail; she could request it via e-mail as well. Certifying authorities may provide directory services; if Bob works for company Z, she could look in the directory kept by Z's certifying authority. Directories must be secure against tampering, so that users can be confident that a public key listed in the directory actually belongs to the person listed. Otherwise, they might send private encrypted information to the wrong person.
Eventually, full-fledged directories will arise, serving as on-line white or yellow pages. If they are compliant with ITU-T X.509 standards (see Question 165), the directories will contain certificates as well as public keys; the presence of certificates will lower the directories' security needs.