Symbolism: Symbolism is when a concrete object stands for an abstract concept. For example, a red rose (concrete) represents love (abstract). There are certain symbols that are fairly recognizable or set, such as a cross being a symbol for a religion. Other symbols are up to interpretation based on the context in which they appear. For example, an apple can represent education (if found in a classroom or given to a teacher) or an apple can represent deception (Adam & Eve story or Snow White). It is up to the reader to interpret the symbol based on context.
Foreshadowing: Foreshadowing is like a hint at what is to come. For example, a dark cloud could foreshadow a storm.
Allusions: Allusions are references to other works. This is often done to describe an emotion or other abstract concept. Common allusions often come from the Bible or mythology. For example, if I called someone "Sleeping Beauty," I am making an allusion to a fairy tale in which a woman is placed under a sleeping curse. This allusion could reference the person's inability to stay awake or reference someone who lacks energy.
Imagery: Imagery is the use of descriptive language to create a picture in a reader's mind using the five senses. For example, I could describe a field of flowers as, "A green pasture blanketed by red, yellow, and orange flowers."