Long Term Memory: memories about the world and life, that are stored and represented in networks in the brain. Long term memories can be retrieved at any given time.
Short term memory: Working memory that holds all the current information for use right now. Only holds an average of 7 memories in one time, and only holds them for a short amount of time.
Source: "Psychology: Second Edition" by Saundra K. Ciccarelli and J. Noland White. (book source)
People tend to more easily store material on subjects that they already know something about, since the information has more meaning to them and can be mentally connected to related information that is already stored in their long-term memory. That's why someone who has an average memory may be able to remember a greater depth of information about one particular subject.
Source: "How Human Memory Works" by Richard C Mohs, PhD
http://health.howstuffworks.com/human-body/systems/nervous-system/human-memory2.htm
It may take children longer to store some things in their long term memory because they have little or no previous knowledge about many subjects, so the information is new to them.
