I will limit my answer strictly to the question regarding the difference between "Pro Se" and "Pro Per." Quite frankly I am stunned by the previous answers. Both of which, while not actually wrong, are not in fact answering the question. To wit; THERE IS a very substantial legal difference between proceeding "pro per" (or "in pro per" or "in propria persona") and proceeding "pro se" (or "in pro se"). Understanding of these two terms is critical, especially pertaining to personam jurisdiction.
There are two types of jurisdiction: "personam" and "subject matter" and a court needs to clearly have jurisdiction of both in order to hear a case for or against you.
Personam jurisdiction gives a court the authority over your person or you as an individual in order to hear or try a case involving you.
Subject matter jurisdiction gives a court the authority over the thing, issue, or activity (i.e. negotiable instrument, car collision, injury to person or property, alleged crime, etc. alleged in a complaint) in order to hear or try a case involving you.
Believe it or not, use of the wrong legal phrase can sabotage you, jurisdiction-wise.
Okay, let's look at the two terms in detail and in depth.
"In propria persona." In one's own proper person. It was formerly a rule of pleading that pleas to the jurisdiction of the court must be plead in propria persona, because if pleaded by attorney they admit the jurisdiction, as an attorney is an officer of the court, and he is presumed to plead after having obtained leave (permission), which admits the jurisdiction. See Pro se. Black's Law Dictionary, 5th edition, pg. 712
"Pro se." For himself; in his own behalf; in person. Appearing for oneself, as in the case of one who does not retain a lawyer and appears for himself in court. Black's Law Dictionary, 5th edition, pg. 1099
Appearing "pro se" is not the same as appearing "in propria persona" as one appearing "pro se" is serving as his own attorney and thereby granting the court jurisdiction (personam jurisdiction) as all attorneys are officers of the court. As you saw in the definition of "in propria persona" above, when attorneys plead for you, they automatically admit the jurisdiction (court's authority). Attorneys are agents of the court that are used to give the court automatic jurisdiction. That's why you are always told to get an attorney for your court action and not usually for the reasons you are lead to believe; in short it's because they are officers of the court and automatically give the court authority over you and this makes things easier for the judge.
If you or any one else wishes to argue the above points I first ask that you obtain a copy of Blacks Law Dictionary Fith edition and look it up for yourself. The Black's Law Dictionary, 5th edition, definition provides us with crucial information, especially if we are to argue jurisdiction. You see, before a court can proceed with an action against you or involving you, be it civil or criminal, jurisdiction (personam and subject matter) must be established on the court's record and this is black letter law.
"The law requires proof of jurisdiction to appear on the record of the administrative agency and all administrative proceedings." Hagans v. Lavine, 415 U.S. 533
"The record must show affirmatively that the jurisdictional requirements have been satisfied." Hayman v. L.A., 17 Cal.App.2d. 674
"Once challenged, jurisdiction cannot be assumed, it must be proved to exist." Stuck v. Medical Examiners, 94 Ca2d 751, 211 P2d 389
"Jurisdiction may never be assumed, not even by colorable claims or status or black robes or officialdom or appearances, but must be substantively proven by the plaintiff/claimant of said jurisdiction. Once challenged by any proper party the plaintiff/complaint must prove their jurisdiction in a timely manner." McNutt v General Motors Acceptance Corp., 56 S.Ct. 502