24K gold or 24 karat gold is pure gold where Karat or Carat is used to define the purity of gold.  1 Karat or Carat corresponds to 1 part of gold in 24 parts of gold alloy. Therefore 24K means it is fully made of gold. Though in theory 24K gold is pure gold, it is generally made up of 99.9% of gold.

Pros
1. Since it is the most purest form of gold it is used to make gold coins and other forms used generally from investment point of view.
2. Used in medicine

Cons
1. Gold in its pure form is brittle and hence cannot be used for jewellery.
2. Since it isn’t hart enough should always be handled with care.