A defensive back is a player who plays in the secondary of a defense. This includes cornerbacks and safeties. Most teams play two safeties and two cornerbacks on the field at once.
There are two safeties on a football team, the free safety and the strong safety. Strong safeties are like smaller linebackers, they have good tackling skills and can rush the passer. Free safeties are better in pass coverage; they read the quarterback's eyes to try and break up a pass.
Cornerbacks offer very little fantasy value outside of interceptions. Cornerbacks make pretty much the entirety of their points from interceptions, which is not a good thing in IDP leagues. Tackles are far more valuable, and the picks are too far in between.
It is easy to get caught up taking a big name cornerback, however, that is not a smart decision. The best cornerbacks tend not to get the ball thrown their way. With that comes less counting stats like interceptions, passes defended, and tackles.
A better choice is to pick a safety, more specifically a strong safety. Strong safeties on average get the most tackles, but there are some elite free safeties that are strong fantasy plays. Safeties are the second best position in defender leagues behind linebackers, and the best safeties are actually as good as many linebackers.
Because most of a cornerback's scoring comes from interceptions, they are a high risk high reward fantasy option.
Safeties are much more versatile racking up points in: tackles, sacks, interceptions, fumbles, and passes defended.
Below is a chart showing the possible point values for defensive players in fantasy. All defenders can earn points for the same things, but we'll take the liberty of narrowing it down to the most common for each position to make things easier. The cornerback's most common source of points are highlighted in yellow while the safeties are in blue. Shared actions are in green.
Safeties earn their points in more ways than cornerbacks, making them a more appealing fantasy option.