Introduction to the "Sensing" Functions

Introverted Sensing: How it works
While Introverted Intuition stores information in the form of ideas and impressions, Introverted Sensing (Si) stores it in a much more literal way. An Si user, when looking back on past events, will remember what actually happened. While an Ni user might remember why someone said something in a past encounter, and what they meant abstractly, an Si is more likely to remember the words that were actually said, and work from there.
Since Sensing functions emphasize what is tangible and more or less immediate, Si's retention of patterns is based strongly on past events. If Si hasn't already experienced something, it doesn't speculate on what it might be like. This is actually often advantageous because Si users have a good grasp on what they do know. It can help them to avoid the intuitive trap of embellishing reality with fantasy.
Si users are often considered the pillars of society. They tend to have faith in establishment and trade in some of their creativity for good old fashion perseverance.

Extroverted Sensing: How it works
While Introverted Sensing deals with memory and past sensory experience, Extroverted Sensing (Se) deals with immediate sensory experiences. Se takes in sensory experiences directly and promotes awareness of whatever environment the Se user is in. A well trained Se gives the user good reflexes and physical dexterity, often making high Se users natural athletes.
Se users can be impulsive because they often take the most direct and immediate course of action. Ni by itself isn't aware of its immediate environment, conversely, Se isn't aware of consequences beyond what is right in front of it.
Se users often like to live life to the fullest. Their sensitivity and immersion in their environment makes extreme activities attractive. Base jumping and sky diving are extreme examples but sometimes it can be as simple as a bike ride or a football game.