When we cover introductory seismology, we learn about P, S and maybe L waves. But the travel time curves a couple of blogs down show many more "phases," with acronyms like PKIKP. Yikes, what is that letter salad all about? Well, the letters indicate where the seismic wave has traveled in the Earth, and whether it has traveled as a P or an S wave, or an L wave if on the surface. The trick is that as waves are reflected from interfaces, like the boundary between the Earth's mantle and core, or when they are bent (refracted) as they cross these boundaries, P waves can convert to S waves and vice versa.
So, we can get phases like those shown below:
These figures are from the text An Introduction to Seismology, Earthquakes, and Earth Structure, by Seth Stein and Michael Wysession, Blackwell Publishing, 2003. (I just discovered this online resource - it's great.)
To translate what the letter sequences mean, there is a full listing of seismic phases agreed upon by IASPEI, the International Association of Seismology and Physics of the Earth's Interior.
C U soon!