The Levels of Processing Effect (Lockhart and Craik) might suggest that we forget because we have not processed understanding on a deep enough level. We may have memorized something at a surface (or shallow) level, but for lack of making a deeper cognitive connection (such as applying knowledge in a new way) we forget the new piece of information.
The Atkinson-Shiffrin theory is more familiar. They proposed the idea that humans store new information in our sensory, short term, or long term memory banks. Sensory memory lasts only long enough for one of our five senses to process what were are seeing, hearing, smelling, etc. Short term memory kicks in when a piece of information is used, usually immediately (such as dialing a phone number). Long term memory is when information goes from storage for a few minutes, to storage for a lifetime. We are much more likely to forget things stored only in our sensory or short term memories, whereas long term memory banks store information that we can recall, hopefully, for life.