jeff28 wrote:2. What does training in Dex do for you? What does dex affect?
As people have mentioned, Dexterity and Will increase your focus capacity and regen, respectively.
If you go into the player information window ('P' key by default), you'll see two columns of characteristics. In the first column are your current values for HP/Sap/Stamina/Focus. It shows the capacity and below that it shows the regen rate. In the second column, you'll see the 8 different characteristics you can train it at the vendors (strength, will, balance, etc.). Each of those characteristics increases the stat that's on the same row in the interface.
In addition to affecting the levels of your bars, these attributes also control what equipment you can use. Heavy and medium armor is based on constitution, melee weapons are based on strength, magic amplifiers are based on intelligence, and range weapons are based on balance. In general, the level of the item you can use is the attribute+10. So if your intelligence is 30, you can use a Q40 magic amp. Medium armor, however, is strength*1.5.