A theme coded for the WordPress content management system that costs money.
At a basic level, it’s easy to understand that a premium WordPress themes costs money and a free WordPress theme does not. Then the question becomes, what do you get for that extra money?
While there can good-looking free themes and poorly-coded premium themes, it stands to reason the two groups will tend to have different outcomes due to their different incentive structures.
Premium themes will be a more attractive option for many professional designers. This means the premium theme market tends to contain a lot of highly original, fully-featured designs.
Beyond the initial quality, premium themes have a better incentive structure for provide quality support and ongoing updates, which can be important considerations for businesses.
What To Consider When Choosing A WordPress Theme
Smashing Magazine (December 4, 2014)
How To Get The Most Out Of A Premium WordPress Theme
Smashing Magazine (February 8, 2013)