The CCNA is an entry-level, single-exam certification that establishes basic knowledge of TCP/IP protocols, ports, services, plus router and switch settings, configuration, and best practices. The CCDP requires the CCNA as a pre-requisite, and also requires passing 2 or 4 exams (the 2 exam option sounds better, and costs $100 less, but basically covers the contents of 3 exams from the 4-exam version in a single, long, arduous exam). CCNPs are more senior than CCNAs, and usually have more experience and thus also make more money.
Because you can't get a CCNP without first getting a CCNA, you can't be in the situation where you have a CCNP but not a CCNA. Thus, my points about experience, knowledge, and pay are entirely correct and explain the junior (CCNA)--senior (CCNP) relationship between the two credentials.
I hope this answers your questions.
This was first published in October 2001