The conservative setting will increase bids slower when testing and decrease faster when there is any indication that a keyword is not working. This is best suited for the ACoS conscious sellers.
The aggressive setting will make bigger bid increases to try and get traffic to judge keyword success faster but at the cost of a higher ACoS in the short term. Aggressive campaigns will typically try a lot more search terms than conservative but will have a larger short term profit hit during the learning phase, which is usually a week or two. This is best suited for sellers who dont mind a slightly higher ACoS to achieve more sales.