Think of skills as power-ups. Each skill provides a specific benefit to you. You can have a maximum of 75 level for each category (the exception here is a Tycoon skill which has a maximum level of 500).
Auctioneer - each level lets You take part in one more auction at the same time..
Tycoon – each level gives you additional 5 property slots. You start the game with 1 skill point which gives you portfolio space for 5 properties.
Lawyer – each level point allows you to have 1 more property with the paperwork undergoing at the same time. You start the game with 1 point.
Innovator – each level reduces costs for all of your properties.
Landlord – each level boosts rent for all of your properties.
Accountant – each level reduces time needed for paperwork. After you purchase it, it will not apply to "pending" paperworks you have at the moment, only for new ones.
Speculator – each level boosts your Empire value.