This is either a single- or double-hung window, depending on how many of the sashes are operable.
A just looks like the wall framing header. There's often a two-by laid flat along the bottom and members on edge above, but not always.
B is the original window jamb (frame), or an extension. Sometimes window and door jambs aren't deep enough for the walls they're in.
C is the lower jamb, or sill. It's sloped, which is why the side jambs (B and F) extend below at the inside.
D is the head jamb.
E and F appear to be stops, which are what retain the sashes. Sashes are the moveable (operable) or stationary frames that contain the glass.
G looks like a trimmer stud, but it's hard to see. They're studs that support the header. Ordinarily they don't stop at the framing sill, but things vary.
H might be a support block. Not sure about that or I. Neither are likely important in your case.
J is the framing sill. It's a horizontal member to which new window flanges might attach, and it supports siding, drywall, and trim.
K and L are brick mold, or casing. It covers the gap between framing (sheathing) and the jamb.
M is the outside edge of the sill mentioned earlier.