For a well rounded group in all four roles, and 3 different power sources (Arcane, Martial, 2 Primal):
Striker: Rogue, DEX/CHA build. Skills: Acrobatics, Bluff, Intimidate, Stealth, Streetwise, Thievery.
Leader: Bard, CHA/INT build. Skills: Arcana, Diplomacy, History, Religion, Heal.
Controller: Druid, WIS/CON build. Skills: Endurance, Heal, Insight, Nature.
Defender: Warden, STR/WIS build. Skills: Athletics, Dungeoneering, Nature, Perception.
Result: All 17 skills covered, with some overlap on Heal (with the Druid being the true person for the Heal check, the Bard picking it up for emergencies despite not having WIS primary/secondary), and Nature (shared between Druid/Warden.) Hits all four roles. And likely has a bit of fun in the process. ;)
Being trained gives you access to options for certain skills. Aside from these there is no difference in what you can do untrained compared to trained, assuming you can make the required DC skill check (clearly easier to do if you are trained).
Trained only abilities:
- Acrobatics - Reduce falling damage
- Arcana - Detect Magic, Monster Knowledge
- Dungeoneering - Monster Knowledge
- Nature - Monster Knowledge
- Religion - Monster Knowledge
- Thievery - DM Option to declare certain tasks so specialized as to require training.
Best Answer
This quote is from Dark Heresy, which, along with Deathwatch, are the only books i own, but being the same rules system, i believe this should be the answer you seek:
So, those skills which you improved further after the game has started, through talents or abilities, are Skill Masteries. But the ones you start the game with are simply skills you have the necessary training to use them.
So, you gain the ability to roll checks with no penalty with your starting skills.
But, during character creation, you can spend the initial experience points to buy those skills again and thus recieve the +10 advancement from skill mastery.