A grapnel could be a type of anchor. From the Wikipedia entry on anchors:
A traditional design, the grapnel style is simple to design and build. It has a benefit in that no matter how it reaches the bottom one or more tines will be aimed to set. The design is a non-burying variety, with one or more tines digging in and the remainder above the seabed. In coral it is often able to set quickly by hooking into the structure, but may be more difficult to retrieve. A grapnel is often quite light, and may have additional uses as a tool to recover gear lost overboard; its weight also makes it relatively easy to bring aboard.
Grapnels rarely have enough fluke area to develop much hold in sand, clay, or mud. It is not unknown for the anchor to foul on its own rode, or to foul the tines with refuse from the bottom, preventing it from digging in. On the other hand, it is quite possible for this anchor to find such a good hook that, without a trip line, it is impossible to retrieve. The shape is generally not very compact, and is difficult to stow, although there are a few collapsing designs available.
A grapple, or grappling hook, consists of metal claws on the end of a rope, used to pull a vessel close prior to boarding.
There appears to be some overlap between the two words, grapnel and grapple.