Softballmom,
When a play is being made at a base- any base, not just home- and the fielder already has the ball, the runner has two options. She may either slide, or remain on her feet but avoid contact with the fielder.
If she slides, she may legally contact the fielder withour penalty (as long as her slide is a legal one and she does not try to, for instance, kick or injure the fielder).
If she remains on her feet, she may slow down, stop, go to the side or otherwise attempt to avoid contact. If she does none of the above, and runs into the fielder she is out for interference.
If the umpire judges that she crashed into the fielder with diliberate force, in an attempt to dislodge the ball or injure the defender, she is out and ejected for malicious contact.
That is the scenario when the fielder has possession of the ball and is making a play. If the fielder does not have the ball, the runner may remain on her feet and continue directly to the base. If there is contact, the defense is now at fault and you may have an obstruction call and the base awarded to the runner.
Even when the fielder does not have the ball, the runner does not have the right to purposely crash into her with the intent to injure or disrupt the play. That would also be malicious contact and an ejection.
One more twist on this play: If the defender gains possession of the ball, then quickly moves into the path of the runner, such that it is impossible to avoid her, the runner may contact the fielder, as long as she does nothing else that might be deemed malicious (throws an elbow, violently pushes the defender, etc.).
On plays at any base, the runner has the responsibility to slide or avoid contacting a fielder holding the ball and the defender has the resposibility not to obstruct the runners path if not in possession of the ball. All players have the responsibility to not maliciously contact an opponent.
There's a lot of judgement involved for the umpire on these plays and he has just a few split-seconds to soak it all in and apply the proper ruling.