A latch is an internal Oracle mechanism used to protect data structures in the SGA from simultaneous access. Latches are more restrictive than locks in that they are always exclusive. Latches are never queued, but will spin or sleep until they obtain a resource, or time out. Enqueues and locks are different names for the same thing. Both support queuing and concurrency.
Semaphores are an operating system facility used to control waiting.
Oracle initialization or INIT.ORA parameters with an underscore in front are hidden or unsupported parameters. Some of these events are version specific, so please call Oracle before using any of them. You can use the oerr command to get the description of an event. Sure there are, but it is hard to find them. This table might help you to read those dreaded trace files and internal messages.