**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 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.