Holding is one of the direct free kick fouls as outlined in Law 12. There are different types of holding but they have changed in the modern game. Players have become more sophisticated in their play and largely due to coaching and watching game on TV.
What we look for in all decisions, is that unfair advantage. Although players may be holding, does it affect the play?
In the picture below, this is a discreet hold by the player in black. Referees need to be aware of fouls like this. Referees should be aware that when 2 player are vying for the ball, fouls like this are frequent.
This next picture is more obvious. Desperation by the player in red makes this an easy call. Fouls like this the referee should be thinking about misconduct as this is blatant holding. And if this foul breaks up a promising attack, then the referee must caution.
This is the type of foul that referees want to see how the play is developing. Yes, there is holding, but the player in black is shielding so we want to wait until there is an unfair advantage gained. BUT, if the player in white gets more aggressive, then the referee needs to blow the whistle for the foul.