Kar 88 was the German service carbine from 1888 to 1898, held in reserve and wartime emergency reissue for WW I and WW II.
V.C. Schilling was the manufacturer in Suhl, Germany; year 1890.
Original caliber was 7.9x57J for a .318" roundnose bullet
"S" marking means that the chamber neck and throat were reamed for use with the 7.92x57JS high velocity .323" spitzer bullet that Germany went to in 1905.
All lumped together these days as 8mm Mauser.
BUT those old guns are not really strong enough for the more powerful "S" cartridge. They were converted for use by reserves and support troops who didn't do much shooting under the theory that if one gun in a company blew up, at least the rest of the company HAD rifles.
You will hear different recommendations:
1. It is old, don't shoot it at all.
2. It is really a different caliber, shoot only with proper 7.9x57J (8x57J) .318" bullet ammunition... if you can find any. I found this but don't know if it is a current offering:
http://www.qual-cart.com/8x57j.htm
Me? I handload and could produce the ammo but it would take some scratching to find the .318" bullets even so.
3. Modern American 8mm Mauser ammunition is loaded with a 170 grain .323" bullet but it is based on the old 8mm Remington Special introduced after WW I and has a thin jacket, soft core and is loaded to low pressures so as to be usable in the old guns. It is supposed to squeeze down in the J barrels and still fill the rifling and shoot accurately in an S barrel. But see 1 and 2 above.
4. Do NOT shoot with the current crop of Turkish and other European surplus, that stuff is all S bore and is loaded hot.
The rifle has an internal magazine. It originally used an en-bloc clip that held five rounds, was loaded from the top and dropped out the bottom opening when empty. Some of the rifles with the "S" conversion were altered to pop the empty clip out the top like a Garand and the hole in the bottom of the magazine covered; some were further altered to load from a '98 Mauser stripper clip and will have clip slots at the rear of the receiver opening. I don't know where to get '88 clips, either.
Somebody will come along and tell you why not to shoot it anyhow.