The most common source of permanent bonuses to your Skills will be through Career Advances. It costs 100 XP to advance a Skill by 10% (WFRP Core Rulebook, Page 27). Advancing a skill in this way will always further your bonus - from 10% to 20%, 20% to 30%, etc. - but the advance can only be taken if it is a skill your career has access to. In other words, getting the +20% requires Exiting your Career into a second Career that has the same Skill available as an Advance.
Talents provide bonuses in specific instances and are meant to represent areas of expertise for a given character. For example, if you have Etiquette (+10% to Charm while dealing with nobility) and Schemer (+10% to intrigue-related Charm tests) your character would receive the benefits of both Talents while dealing with intrigue specific to Nobles.
In Combat all bonuses are cumulative unless otherwise stated. In your example the bonuses do stack, as long as the Opponent attacks with a Standard Attack (since Aim only applies a +10% to your next Standard Attack). A handy thing to pay attention to: specific Actions that grant bonuses (like Aim, Guarded Attack, and All Out Attack) will list the circumstances under which they provide those bonuses. Aim does not stack with All Out Attack because All Out Attack is not a Standard Attack. Similarly, two Attacks will not stack because only one Attack can be made in a turn (So you cannot Charge Attack and then Standard Attack, even if you have been buffed with Stag's Leap).
For Magic Items, Maybe
Magic Items in Warhammer Fantasy are as powerful and coveted as they are rare and mysterious. Bonuses provided by these powerful artifacts does stack with bonuses your character has earned through Advances.
If you are fortunate enough to come across two Magic Items that grant a bonus to the same skill AND your party agrees that you should be allowed to have both, it is technically true that the bonuses would stack, however this could lead to a scenario where your Skill bonus reaches a ridiculous level.
As far as I know, there is no official ruling on the stacking of Magic Item bonuses (possibly because the writers did not expect such an unlikely scenario to come to pass) but my group has always deferred to the GM's best judgement in such cases.
Again, these instances are very rare; on one occasion requiring careful coordination between two players taking advantage of the fact that our GM wanted each player to have access to a "custom" Magic Item - both players requested Magic Items with similar properties and would allow the other to borrow the item essentially doubling all of the bonuses. The GM eventually ruled that the bonuses wouldn't stack for that instance since it was clear the two players were intentionally gaming the system.