There are many commercial CAs that charge for their services. Institutions and governments may have their own CAs, and there are also CAs which are free of charge.
If the user trusts the CA and can verify the CA's signature, then he can also verify that a certain public key does indeed belong to whoever is identified in the certificate. If the CA can be subverted, then the security of the entire system is lost.
Suppose an attacker, Mallory (to use the Alice and Bob convention), manages to get a CA to issue a false certificate tying Alice to the wrong public key; the corresponding private key is known to Mallory. If Bob subsequently obtains and uses Alice's public key in this (bogus) certificate, the security of his communications to her could be compromised by Mallory - since Bob's messages could be decrypted by Mallory, or Bob could be tricked into accepting signatures which are forged to appear to be from Alice.
In large-scale deployments, Alice may not be familiar with Bob's certificate authority (perhaps they each have a different CA), so Bob's certificate may also include his CA's public key signed by a different CA2, which is presumably recognizable by Alice. This process typically leads to a hierarchy or mesh of CAs and CA certificates.
Worldwide, the certificate authority business is fragmented, with national or regional providers dominating their home market. This is because many uses of digital certificates, such as for legally binding digital signatures, are linked to local law, regulations, and accreditation schemes for certificate authorities.
However, the market for SSL certificates (used for website security) is largely held by a small number of multinational companies. This market has significant barriers to entry since new providers must undergo annual security audits (such as WebTrust for Certification Authorities) to be included in the list of web browser trusted authorities. More than 50 root certificates are already trusted in the most popular web browser versions. A 2007 market share report from Security Space as of September of that year determined that VeriSign and its acquisitions (which include Thawte and more recently Geotrust) have a 57.6% share of the certificate authority market, followed by Comodo (8.3%), and GoDaddy (6.4%). According to Netcraft's July 2008 Secure Server Survey, additional top providers include GlobalSign and Entrust. Netcraft's findings do not include non-public-facing certificates.