Longganisa is the Filipino version of longaniza which is a Spanish sausage. The sausage may be made of pork, beef, chicken and there is even a version made with tuna. The names and flavors change depending on the region from which it is made. In some regions of the Philippines the sausage may be referred to as Chorizo. This sausage is typically served for breakfast along with rice and eggs.
When shopping for longganisa (or making your own) you can find it stuffed in a casing like most other sausages (usually short, plump links) or hand formed into skinless links or simple patties.
If you are here in the U.S. you probably won't find these sausages locally. If you happen to have a store that specializes in Philippine foods you can look for it there. Otherwise you can purchase it online (frozen) from Filstop.com.
Recipes for longganisa are plentiful on the web. It's not difficult to make particularly if you make it in hand formed rolls or as patties.