What is the difference between a Specialist and a Corporal in the Army? Inquiring Marine minds want to know.

1stSgt,
there is really no considerable difference between the 2 ranks. They are the same pay grade but one, the CPL, is considered and NCO. The Specialist can be in charge of others at the same rank, typically a team leader. As for me, I have been both, more than once, but that's a story for another day.
In the Army, a CPL can have the same responsibility as a SGT and be held to a higher standard. These days, I have seen a lot less CPLs, but it is a good stepping stone to NCO Corps. As for getting promoted to CPL, unless it comes from Dept. of the Army, it is typically a lateral promotion and once you leave that certain unit that you got promoted in, you lose that promotion, unless the gaining unit needs that specific position. I hope this makes sense.

I assume you're not in the U.S. Army, but this is a good question. Responsibility, leadership position/billet, and at the Commander's tool to promote leaders due to potential. This rank stays "within" the unit in so many words. This is not verbatim from Army Regualtion, but it should cover it. I don't see many Corporals, but when I do, they do stand out.
Our Specialist rank is very common due to the promotion system, available leadership positions, time in grade, and other reasons. There is a difference, in these two ranks. I had a blast being a Specialist! I had enough knowledge to be an NCO, but the excuses in case of mishap to be a Junior Enlistedman. Love it! When in pinned Sergeant, that all went away.

Nco

As to pay plans I don't think there is any difference. The main difference is that an SPC is more likely than not to be in a technical position like Missile Systems ( That's a broad generalization ) where a corporal would be more suited for infantry, armored cavalry or artillery units. Promotions for Spec 4 from PFC and to Spec 5 from a Spec 4 may be quicker than the straight E-XX.
Yes, both sides are governed by mandatory TIS and TIG but specialist units may have a Proud Papa Battalion Commander who gets his way with the promotion boards. At least, that's been my experience ( SP-5 right before my 5 year mark ) and my observations.
I made rank quickly as well given my Eyes Only Security Clearance in Germany, just because there was a cipher machine in the teletype room. And our resident Full Bird loved to hand out promotions to the only enlisted person in the unit. For the duration of my position as a forensic chemist there at CID HQ for EU / Asia I carried creds of a W-2. ( I was plainclothes too ).
It all just depends on when you are where you are AND your performance. All IMHO!

A SPC is basically a senior/junior enlisted Soldier who's been around for at least 2 years. A CPL was a PFC or SPC that the leadership noticed a great deal of leadership qualities and potential in and decided to give them stripes so that they can lead (no "official" NCO board required). The SPC rank has been around for quite sometime, and a Soldier use to have the option to either stay a SPC and move up the pay-grades or go the NCO route and move up the same way, but was a "hard striper". They have since of course removed the SPC 4's,5's, 6's, 7's-9's and the product you see before you today is the descendant of that. Being a former Combat Engineer, we had a few CPL's running around. For the most part, CPL's are utilized in Combat Arms specific MOS's (Military Occupational Specialties). I have NEVER seen a CPL in a non-combat arms unit, unless the promotion was a permanent one granted they re-classed from Combat Arms. You will never meet a CPL cook, HR specialist, Intel Analyst, and so on. CPL's are the mean little bastards that are needed in grunt and other Combat specific units. The Army decided to throw CPL stripes on my chest for the last 3 months of my first enlistment when I was a Combat Engineer before I got out back in the day. Had to take them off when I came back in because I had no orders for the rank, was just requested to be a CPL. Here are a couple pics from our old enlisted rank stuctures

Cpl the most laughed rank in the army!

They hold the same rate, but one is a NCO and the other is not.

The ONLY difference is that a Spc is not in a leadership position and a Cpl is Kinda.... A Cpl has more responsibility and gets more stupid ( back in the day anyways) jobs that wouldnt normally befall a Sgt . Used to be that a Cpl didnt want to hold the rank because of the nonsense that went with it. Fire Guard for instance, back then being a Cpl was not a ticket to get out of it but an open invite to pull as much of it as Top saw fit because he needed his "real" Nco's during the duty day. same thing went for other duties where a "real" Nco was needed, police call...Hey CPL !!!!!... Dining hall duty...Hey CPL !!!! and so forth...I myself think that if they wear it they should be treated as it was intended to be...that person had demonstrated above and beyond and was promoted to an NCO before they made the cut off score.

The one thing I have to add to this conversation relates to the permanence of the position. After I was "promoted" to Cpl I held a Section Chief (Squad Leader) position. After a couple of years, a Ssg transfered into the unit. I assumed that this was the end of my position and swapped out my rank with the sham shield. I was chewed out by the PL for being out of uniform. We quickly went to my file where I saw that my rank had been permanently changed. If I remember right, my 214 rank is E-4/Cpl.

Having served in the Army, Army National Guard, and Marine Corps, I can say that almost everyone is correct, to some degree. A "Hard Striper (not stripper ) Leads as part of overall Command Structure... At All Times and in All places. A specialist has specific skills, and "Leadership Skills" may not be as important as the skills they possess. In "their" work place there is no difference per se. Example: If a Corporal was assigned to Mess Hall/Chow Hall duty, the Spec-4 running the team in the Chow Hall will reign supreme over the Corporal. (Two things determine seniority: 1. Rank/Grade AND 2. Billet position... Billet POSITION while acting in the capacity of that position TRUMPS over a senior grade acting in a Junior position.