Think of the inside centre as a fly-half playing in midfield. In New Zealand they call them second five-eights.
But the major difference between the two positions is the players' body composition. Inside centres tend to be bigger - and a bit quicker - than fly-halves.
This means they are able to take the ball into contact (tackles) more than a number 10 normally would.
Modern day players are now leaner and stronger because the modern game is physically demanding.
Farrell can put wide passes in, he can put short passes in or take the ball into contact
But that doesn't mean modern day centres use brawn over brains.
Mathew Tait or James Hook don't try to smash through players when they play at 12. It's about using your attributes to the best of your abilities.
However the way centres play is very subjective and comes down to the individual coach's tactics for that game.
For example, take a look at Andy Farrell at inside centre. He's a guy who can distribute the ball, but he's not quick.
He has good tackling skills because of his rugby league background, but is seen as someone who can play the ball with his hands.
Farrell can put the wide passes in, he can put short passes in, or take the ball into contact and pass the ball out of the tackle.
In theory the ideal skills for a number 12.