A rectangle, with the origin at the upper left (integer).
Defined in SDL_rect.h
SDL_Rect srcrect;
SDL_Rect dstrect;
srcrect.x = 0;
srcrect.y = 0;
srcrect.w = 32;
srcrect.h = 32;
dstrect.x = 640/2;
dstrect.y = 480/2;
dstrect.w = 0;
dstrect.h = 0;
extern SDL_Surface *src, *dst;
SDL_BlitSurface(src, &srcrect, dst, &dstrect);