user7312: The USCF rating system as described by Professor Glickman includes a factor called K:

If m < 3, or if the player competes against any opponent more than twice, the "standard" rating formula that results in Rs is given by
Rs = R0 + K(S - E)  

(source, emphasis mine)
Though at one point the K value was calculated for each player from the pre-event rating, it is now defined by one of two expressions depending on whether the tournament is "half-K" or "full-K:"

The value of K, which used to take on the values 32, 24 or 16, depending only on a player's pre-event rating, is now defined as...

(source: same page as above)
What does it mean if a tournament is half-K (besides that it changes the value of K)? Why would the tournament director choose to make an event half-K?
I also see that the USCF considered removing the half-K option, then did remove it, then considered using it for quick games in some way. However, none of those reports explain what being half-K actually does to a tournament.

user9030: As dfan mentioned in his comment, K factor, is a factor for changing convergence speed of ratings. When you win against a strong player, your rating converges to your opponents rating, after some consecutive wins, you will have the same rating with your opponent. K factor simply changes how many games you need to converge we can say. Situations where it is used:

High K used, in first few matches of a player, to let the player reach actual Elo quickly
Low K used, in some tournaments which has non-standard time controls
Low K used, in matches between masters, to stabilize Elo changes.

etc. 
There is no absolutely correct K value, it's really open to question how fast it should converge. But when it converges slower, Elo points are more stable. When it converges faster, people can reach their actual ratings faster. 
I caused a lot of trouble to people just because of this dilemma. I had an initial rating of 1100(for some riddicilous reason, long story). And at the time my actual strength was about 1500. In time, my strength went up to 1900 but my rating was about 1400. Just because of this, my opponents lost lots of points(unfairly). I remember a game when I was 1300 something, and I won against a 2000 player. He lost a lot of points, and was very sad. If I were 1900, penalty would be much more less.

7312: <p>The USCF rating system <a href="http://math.bu.edu/people/mg/ratings/rs/rs2.html" rel="nofollow">as described by Professor Glickman</a> includes a factor called <code>K</code>:</p>

<blockquote>
  <p>If m &lt; 3, or if the player competes against any opponent more than twice, the "standard" rating formula that results in R<sub>s</sub> is given by</p>
  
  <p>R<sub>s</sub> = R<sub>0</sub> + <strong>K</strong>(S - E)  </p>
</blockquote>

<p>(<a href="http://math.bu.edu/people/mg/ratings/rs/node5.html" rel="nofollow">source</a>, emphasis mine)</p>

<p>Though at one point the <code>K</code> value was calculated for each player from the pre-event rating, it is now defined by one of two expressions depending on whether the tournament is "half-K" or "full-K:"</p>

<blockquote>
  <p>The value of K, which used to take on the values 32, 24 or 16, depending only on a player's pre-event rating, is now defined as...</p>
</blockquote>

<p>(source: same page as above)</p>

<p>What does it mean if a tournament is half-K (besides that it changes the value of K)? Why would the tournament director choose to make an event half-K?</p>

<p>I also see that the USCF <a href="http://www.glicko.net/ratings/report05.txt" rel="nofollow">considered removing the half-K option</a>, then <a href="http://www.glicko.net/ratings/report06.txt" rel="nofollow">did remove it</a>, then <a href="http://www.glicko.net/ratings/report09.txt" rel="nofollow">considered using it for quick games</a> in some way. However, none of those reports explain what being half-K actually does to a tournament.</p>
 9030: <p>As dfan mentioned in his comment, K factor, is a factor for changing convergence speed of ratings. When you win against a strong player, your rating converges to your opponents rating, after some consecutive wins, you will have the same rating with your opponent. K factor simply changes how many games you need to converge we can say. Situations where it is used:</p>

<ul>
<li>High K used, in first few matches of a player, to let the player reach actual Elo quickly</li>
<li>Low K used, in some tournaments which has non-standard time controls</li>
<li>Low K used, in matches between masters, to stabilize Elo changes.</li>
</ul>

<p>etc. </p>

<p>There is no absolutely correct K value, it's really open to question how fast it should converge. But when it converges slower, Elo points are more stable. When it converges faster, people can reach their actual ratings faster. </p>

<p>I caused a lot of trouble to people just because of this dilemma. I had an initial rating of 1100(for some riddicilous reason, long story). And at the time my actual strength was about 1500. In time, my strength went up to 1900 but my rating was about 1400. Just because of this, my opponents lost lots of points(unfairly). I remember a game when I was 1300 something, and I won against a 2000 player. He lost a lot of points, and was very sad. If I were 1900, penalty would be much more less.</p>
