Many experts have studied psychology and human memory, numerous have agreed that there are 3 stages of memory: Encoding, Storage, Retrieval.
1. Encoding Encoding is the first step in the brain's ability to process memory. When an event takes place or information is obtained, the brain instinctively memorizes this through the body's perception of its senses. The 3 main ways the memory is collected is through visual, acoustic, and semantic methods (simplypsychology). Subconsciously, as the body's senses collect the information in order to create the memory, your brain also attempts to associate them with emotion, since emotion can increase one's attention (human-memory.net). This also means it is impossible to have memories with the complete absence of emotions. Your body naturally associates its memories with emotions. In addition, stronger emotional connections with the memory, the stronger the memory will hold inside your brain.
2. Storage Next comes memory storage, which focuses on where the memory is stored and how long it lasts. The importance of storing an emotion is because it directly affects how the memory is retrieved. There are two main types of, being short term memory and long term memory. It is believed the brain has an unlimited amount of "spaces" for long term memory but limited for short term (simplypsychology). However, long term memories often originate from short term memories, through consolidation. Scientifically speaking, best stated by human-memory.net, "Long term memories are stored throughout the brain as groups of neurons that are primed to fire together in the same pattern that created the original experience, and each component of memory is stored in the brain area that initiated it. Indeed, it seems that they may even be encoded redundantly, several times, in various parts of the cortex, so that, if one engram (or memory trace) is wiped out, there are duplicates, or alternative pathways, elsewhere, through which the memory may still be retrieved." In a dumber version: The analogy of memories being stored as "books" in the "library of our mind" is exceedingly incorrect. Moreover, it's that our memories are statues that must be constantly "resculpted" or "reconstructed" through our senses, emotions, and elements of the brain.
3. Retrieving Finally, is memory retrieval, which is recollecting, reassessing, or recalling one of the encoded memories that you've stored. Or in simplistic terms, "remembering". During memory retrieval, the brain repeats the neuron's pattern of activity that was first generated in response to the encoding of the memory (human-memory.net). However, it can occur as well as be classified in numerous ways. Unfortunately, I will only touch upon a few.
First, the classifications. One major one is interference with memory retrieval, either proactive or retroactive. This occurs because of the interference of recollection of the memory, due to the questioning information (lumenlearning.com). Proactive inference occurs when past memories affect the ability to obtain new memories (like learning your new address only to think of your old one) versus retroactive inference happens when you recall a memory, only to have new information presented, making you question the nature of your memory (lumenlearning.com). Another is free recall, where memory can be remembered but in any order and out of order. Therefore, if someone remembers the ending of the memory better, they can tell the memory backward. Lastly is retrieval failure, where a person cannot retrieve a memory successfully (lumenlearning.com). Many know this as, "scratching the surface" or "on the tip of my tongue". Often times this can result from neutron decay, a process where the neutron connections and activity declines (or, as to reference an earlier analogy, the lack of resculpting the memory sculpture).
Now, when a memory is retrieved. Typically, memories are retrieved during recognition, when the brain records, encounters, or witnesses an experience, object, or event that has information relating or comparing to a memory (human-memory.net). When memories are retrieved during this time, it is normally subconsciously. The second time a memory is retrieved is through recalling, where the brain consciously attempts to remember information that requires digging through and uncovering information in the memory, in the lack of clues and information being physically present (human-memory.net).
Final Thoughts & Notes: Memory is fickle. It is considered to be reliable in certain aspects and completely preposterous in others. However, it is important to know that ALL human memory is highly susceptible. For example, "cross-racing bais" is "a reduction in accuracy when identifying faces of a race or ethnic background different from one's own" (ncbi.nlm.nih.gov). Dishearteningly, this occurs a lot. Furthermore, when resculpting and reconstructing our memories, our brain subconsciously adds filters to the memory created from past knowledge, experiences, and events (ncbi.nlm.nih.gov). Therefore, even if an event if remembered one way, as a person's life goes on and the memory is resculpted, the shape can change. Best said by Oliver Sacks, "Every act of perception, is to some degree an act of creation, and every act of memory is to some degree of imagination." In conclusion, it's important to know that human memory isn't perfect, nor is it 100% reliable. So be careful how you share your memories.
There's actually a few other pieces of information to deepen this that would be cool, especially more about how the human memory is flawed. Anyway, if you'd like to learn/read more go for it. Or perhaps, I make a part 2?