If you are operating as a sole trader you have almost complete control over how the business is run. You can make decisions (as long as they are legal!) without interference.

The administrative costs of running a sole business are small.

Cons

All your personal assets are at risk if the business fails. Personal bankruptcy can occur. (Николаенок)

Partnership (Self-Employed)

A partnership is a business run by two or more people together. There should be a written agreement detailing this arrangement. Profits are usually shared between partners according to the agreement. Although profits may be shared unequally, liabilities which may arise are shared jointly. This is something that everyone involved should be very clear about. Even if you only own 1% of the business you will still be responsible for 100% of the liability. (Лазук)

Limited Company

Unlike a Sole Trader or a Partnership, the Limited company is legally a separate entity in its own right. The directors and shareholders have limited liability. When a limited company is created it will have an Authorised Shareholding which specifies the limit of a shareholders liability. If all shares have been issued then shareholders are not liable for any more debts that the company may accrue. This is definitely the most sensible option if capital is being put into the business by anyone who is not involved in running it. (i.e. shareholders) (Титорович)

Pros

Limited liability can usually protect directors, who act in good faith, from legal actions brought against them.

Cons

There is considerably more administration involved in running a Limited Company than there is for a Partnership or Sole Trader.

As mentioned above, even with no other staff, the "owner" or director of the company is considered to be an employee of the company. (Лавренов)