From OpenCog

The GetLink is an older and discouraged version of MeetLink. It performs exactly the same search as MeetLink, except that it returns all of its results in a SetLink instead of a QueueValue.

The GetLink is discouraged because it has proven problematic: when it returns search results in a SetLink, those SetLinks pollute the AtomSpace, and usually they have to be manually deleted after performing the search. The MeetLink avoids this problem. What's more, the MeetLink returns a QueueValue, which is a thread-safe FIFO; thus, it allows one thread to read and process search results, even as other threads are still generating new ones.

See SatisfactionLink and QueryLink for details.