Here's a play that demonstrates the difference between "interference" and obstruction."
Runner on second, no outs. Ground ball is hit to the shortstop (SS). The SS is in the runner's baseline between second and third. The runner at second is going to try for third and makes contact with the SS
before he fields the ball. This is "INTERFERENCE." The ball is immediately dead and the Batter is awarded first.
Same play. This time the SS misplays the ball and had to move forward two steps to pick up the misplayed ball and makes contact with the runner. this is OBSTRUCTION. The ball is dead immediately and the runner is awarded third and the batter is awarded first.
There is an "exception" to this play, which I wasn't going to mention, but someone else may, so I better put it in print. If the fielder, in this case the SS, missed the ball, but may retrieve it with a "step and a reach" he is still protected from the offense (the runner) running into him. If he does, this is still INTERFERENCE. This is why I stated the SS went two steps to retrieve the ball because he is no longer protected once he goes beyond the one step.
Sounds confusing, but after 40 years of umpiring, it comes natural to me. This rule is the same in college and professional baseball.
For pro baseball rules, you can go here..
http://mlb.mlb.com/mlb/official_info/official_rules/official_rules.jsp
For college rules, you may go here.......
http://www.ncaapublications.com/DownloadPublication.aspx?download=BA16.ePub
The dh rule in college is the biggest crock of crap that you will ever read. I think it covers like seven pages in the rules book.
Like Ernie banks used to say, "It's a great day for a ball game. Let's play two."