Boy, Wezz and Razz... confusing
Razz covered it off pretty well, but at the risk of entering the legendary PP territory, thought I'd toss my two cents in. Good coverage of how above, thought I'd be so provocative as to risk mentioning why (or why not).
Transferring from your primary to a secondary vessel allows you to get the beer off the yeast cake. Probably two main reasons for doing this. One is to simply elimanate that source of potentially cloudiness (turbidity) in you finished product. Secondly, once the food runs low, the yeast will start to eat themselves (autolysis). This can produce some undesirable flavours.
As with many things in brewing, there is plenty of debate as to whether this step is necessary. Every time you transfer, there is a risk of infection. Other than 'extra work', infection risk is probably the main thing against secondary. If you are going to let your bit sit around for an extended time it is probably worth doing.
Your secondary vessel should have little or no air space. The 23-litre cubes work well as you can squeeze out extra dead space and then put the cap on. If there is still a bit of fermentation activity, then the cube will expand a bit and you can just burp the gas out once a day or so. This does give you a much better visual indicator of fermentation progress than an air lock as there is less seal to leak. (But SG is probably best indicator)
If you are going to bulk prime, you do need to rack (siphon) your beer from the primary to another container. The reason for this is that you need to mix the priming sugar in and doing this in the primary will re-suspend the yeast and trub.
If you transfer to a secondary, you will probably want to rack again for priming, as more yeast will settle out. So, one more transfer and risk of infection.
I have been doing both. I like the secondary as it has cleared up my beers some (although a good yeast makes a difference too -- have been using US-56 and it cakes up really well). I also like the confirmation of fermentation activity slowing.
I definitely like bulk priming, both because I think you get more even distribution of priming sugar and it is less hassle than filling all the bottles.
Of course, the other way to go is to filter from the primary into your keg and then carbonate with CO2. Just need a bit more gear...