Here is an explanation of cognitive load theory. This was taken from a recent publication of mine:
Pastore, R. (2012). The effects of time-compressed instruction and redundancy on learning and learners’ perceptions of cognitive load. Computers & Education, 58(1), 641-651.
Cognitive load refers to the informational load that is being processed in working memory (Van, Paas, & Sweller, 2010). Cognitive load theory explains that there is a certain amount of information that can be processed in working memory at one time without overloading processing capacity. Thus, when cognitive load is increased beyond our working memory capacity, learning is depressed. Initial research on cognitive load stems from the information processing theory, which explains that we have a limited short term or working memory and an unlimited long-term memory (Brünken, Plass, & Leutner, 2003). Miller (1956) explains that the mind can store seven units of information, plus or minus two units depending on how meaningful they are to the learner, at one time in our working memory without exceeding processing capacity.
Cognitive load is comprised of three types of load that are referred to as extraneous, intrinsic, and germane (Sweller, 2010; see also Künsting, Wirth, & Paas, 2011). Each of these types affects learning separately. Extraneous cognitive load is affected by the design of the instruction. This type of load suggests that irrelevant information results in high cognitive processing. So reducing irrelevant activities will reduce cognitive load and increase comprehension. Intrinsic cognitive load is affected by high element interactivity, such that information that is hard for the learners to interpret increases cognitive load. Germane cognitive load refers to load that is generated by instructional activities that lead to schema development and automation (Mayer, 2005).