Random number-Set.seed(N) in R I realize that one uses set.seed() in R for pseudo-random number generation. I also realize that using the same number, like set.seed(123) insures you can reproduce results. 
But what I don't get is what do the values themselves mean. I am playing with several functions, and some use set.seed(1) or set.seed(300) or set.seed(12345). What does that number mean (if anything)- and when should I use a different one. 
Example, in a book I am working through- they use set.seed(12345) when creating a training set for decision trees. Then in another chapter, they are using set.seed(300) for creating a Random Forest.
Just don't get the number.
 A: In short, the numbers themselves don't really mean anything! If you are looking at someone else's code (like in the two examples you gave above), the numbers don't alter the functionality of the function; neither are there "good" numbers for specific functions. It's just down to the authors' choice. 
Further, if you are only ever setting the seed once in your code, then you can kind of choose any number you like. The only thing you need to be a bit careful of is that, if you interface with any other functions that also use random numbers, then it's good to choose a non-obvious seed (so it's less likely for you both to use the same seed).
However, as Corcovado really nicely points out, for some applications, you need to be really careful about the choice you make. If mathematically you require a number of pseudo-randomly-generated numbers, then there can't be a pattern to the numbers you choose.
A: The seed number you choose is the starting point used in the generation of a sequence of random numbers, which is why (provided you use the same pseudo-random number generator) you'll obtain the same results given the same seed number. As far as your second question is concerned, this short snippet from the description of the equivalent functionality in Stata might be helpful:

We cannot emphasize this enough: Do not set the seed too often. To see
  why this is such a bad idea, consider the limiting case: You set the
  seed, draw one pseudorandom number, reset the seed, draw again, and so
  continue. The pseudorandom numbers you obtain will be nothing more
  than the seeds you run through a mathematical function. The results
  you obtain will not pass for random unless the seeds you choose pass
  for random. If you already had such numbers, why are you even
  bothering to use the pseudorandom-number generator?

http://www.stata.com/manuals13/rsetseed.pdf
A: The set.seed()function in R takes an (arbitrary) integer argument. So we can take any argument, say, 1 or 123 or 300 or 12345 to get the reproducible random numbers. 
Also, in theTeachingDemos package, the char2seed function allows user to set the seed based on a character string.
