Uke (Blocks)
       Uke means "receive." In karate, uke refers to blocking techniques. Blocks are traditionally used to defend against attack and avoid being hit. They are most commonly performed with the arms or hands, and usually precede a counter-attack.  Another important aspect of blocking is that, when applied forcefully enough, they should discourage the assailant from making further attacks, either by causing pain to the attacking limb or utterly destroying it. This is very difficult to actually implement in training since control is of utmost importance and no one wants to go home with broken bones after an evening of practice. Many consider blocks to be of greater importance than attacks simply because karate is a defensive martial art. It should be noted that, although not listed in the blocking section, many of the strikes used in karate can also be used as blocks and vice versa, many blocks can be used as strikes. Uke can be divided into 4 categories:

Basic Closed-Hand Blocks
       Basic blocks are the most common blocks. They are executed with a swinging motion of the arm (up, down, either side) with the fists closed. The point of contact is either the inner or outer forearm, usually closer to the wrist area. Since the entire length of the forearm can be used, they have a much greater safety margin. Therefore, they are the safest blocks to use, especially for a beginner.
Open-Hand Blocks
       Open-hand blocks are performed with the edge or back of the hand, not the arm. Since the hand is much smaller, these blocks have a very small safety margin and are not as common. Usually, open-hand blocks are followed by some form of grabbing with the blocking hand, pulling the opponent off balance, followed by a counter-attack. In many cases, they are delivered as attacks instead of blocks.
knife-hand block (sword-hand block)
vertical knife-hand block
hooking block
backhand block
grasping block (tiger-mouth block)
Advanced Blocks
       Often found in kata, advanced blocks usually have the option of being executed with the hand open or closed. These blocks tend to work best when remaining stationary or when moving forward to meet an opponent's attack, and afterwards, taking the opponent off their feet. These blocks require better timing skills, and are used effectively only by advanced practitioners. Advanced blocks come in all shapes and sizes; only the more common blocks that appear in more than one kata are listed below.
back-arm block
flowing block
pressing block
scooping block
rolling dropping block
Double-Hand Blocks
       Blocks using both hands are usually only practiced in kata. In many cases, double-hand blocks are nothing more than a single-hand block with the other hand touching or supporting it, for situations when more force is necessary. They can also consist of two different single-hand blocks performed at the same time, to block two separate opponents or attacks. Thirdly, two of the same blocks can be delivered at the same time to achieve a synergistic effect, usually occurring in advanced bunkai. Very often, double-hand blocks have special applications that aren't apparent to a beginner or intermediate student. Again, only those double-hand blocks appearing in more than one kata are listed below.

kosa-uke (juji-uke)

double-hand block
cross block (X-block)
wedge block (separating block)
manji symbol block (swirling block, vortex block)
staff block
combined block
scissors block
pushing block
crossing block (inside block & down block)

rising block
outside block
down block (lower level sweep)
inside block
tsukami-uke (koko-uke)
* Although this block is usually referred to as simply a simultaneous uchi-uke and gedan-uke, some instructors have
    called this technique kosa-uke to describe the crossing motion of the arms.