The door is sticking for one of two reasons. Either the door/casing is swelled, or---and more probably the reason, the house has shifted and the frame is racked.

There are two fixes---the quick, cheap one---planing off the part of the door sticking---or the correct long term repair---taking the frame loose and remounting it square in the opening.

Look on all sides to see if the gap is pretty much equal all around the door. If it's fairly tight all around and you can't tell where it's rubbing, slide a scrap of paper all around to see where the rub is. A little settling of the house or seasonal swelling is typically the cause. If you're handy, you can probably plane down the part(s) of the door where you have rubbing. If you have no idea how to go about this, get a competent handyman to tune the door up...it shouldn't be very expensive.

Also, to get the door open and closed before it's fixed, you can often apply strong pressure on the doorknob (downward, or upward, or toward the hinges, depending on where it rubs), and this can move the door a little bit away from the rub spot so you can get it open without yanking directly outward on it.