What is the shark's relationship to a ramora?

No, the remora is not relating to sharks, only things in common is that they are fish, in fact sharks arent there only host, they've been known to attach to manta rays, whales, turtles, and dugong. Here is some information regarding the ramora,

"The relationship between remoras and their perfect hosts is most often taken to be one of commensalism, specifically phoresy. The host they attach to for transport gains nothing from the relationship, but also loses little. The remora benefits by using the host as transport and protection and also feeds on materials dropped by the host. There is controversy whether a remora's diet is primarily leftover fragments, or the feces of the host. In some species (Echeneis naucrates and E. neucratoides) consumption of host feces is strongly indicated in gut dissections.[3] For other species, such as those found in a host's mouth, scavenging of leftovers is more likely. For some remora and host pairings the relationship is closer to mutualism, with the remora cleaning bacteria and other parasites from the host."