Link to original: http://sphynxcatvp.nocturna.org/articles/sc-shielding101.html
What is a "Shield" anyway?
In this usage, "shielding" refers to psi-based defenses around yourself, commonly either within or along the "edges" of your aura/energy field. "Shields" are often called many things by different people, including "wards" or "personal wards", "psi defenses" and so on, all depending on the person's experience and preferences. No matter what you call it, it's still a form of protection from outside influences such as psychic attack and empathic/telepathic overload.
What do you use these "Shields" for?
A common use is to keep other people's emotions, and sometimes thoughts, from affecting YOUR mood. If you pick up on other people's moods automatically - without conscious effort - then learning to shield yourself will be very useful. Otherwise, you can get confused by the mix of other people's emotions, especially in a location where you're forced to interact with others, like school or work. The type of shield best for this purpose is what I call, appropriately enough, a "filter shield" - this is a shield that filters out the components of the energy you don't want, and only lets through what you do want. Think of it like a firewall for your brain. :) You just have to work with it to figure out what works best for you.
Another common use is blocking other people's psi-based attacks. This is a more complex type - people can psi-attack in various ways ranging from throwing energy balls at you to draining your energy, and anything in between. You have to be flexible enough to adapt your shielding methods to the attack being used against you.
What types of Shields are there?
I tend to put them in two VERY basic categories:
Active shielding styles
These need constant attention and energy input to maintain. Attack-blocking shields that attempt to be "solid" 24x7 would be an example of an active shielding style. Because they require continual, or at least frequent, energy input, they are also very noticeable to people who are just casually looking to see if you're shielded. (Compare a person physically walking around with kevlar body armor vs. a person who isn't - the additional bulk is noticeable. Same principle here, even if you're not dealing with anything physical.) Additionally, to a psivamp, an active shield is like hanging a sign on you saying "Free Lunch!" *chuckles*
Passive and Reactive shielding styles
These tend to appear inactive most of the time. Shields that block attacks, but are only activated when you're actually attacked, are an example of a passive shielding style - which in this case, also happens to be reactive. Additionally, because they're not obviously active a lot of the time, they're more likely to be unnoticed by most psivamps. Very much in the "set it and forget it" style. :)
The passive/reactive shield types work best if you maintain constant awareness of what's going on around you - sometimes *just* a reactive style isn't appropriate, if there are cases when it wouldn't react in time. Depending 100% on this type of shield to defend yourself is like walking into a war zone without any armor - you might get lucky, or you might get hit. Being aware of what's going on around you increases your likelihood of not getting hit. :)
There are a lot of shielding principles that fall in each category, but most *types* of shields can use either active or passive methods to create them (not exclusively one or the other - it depends on the individual's personal style), so there's not much point in saying "xyz shield is just an active shield" or "abc shield is just a passive style shield" - the method used will depend on the person creating the shield.