Drawing Interchange Format (DXF)

Wikipedia: Drawing Interchange Format (DXF)

libdxf
LibreDWG
AutoCAD 2008 DXF Reference (PDF)
QCAD – 2D CAD for Windows, Linux and Mac
VisiCut – a userfriendly tool for laser-cutting

DXF File format _ Polyline
AutoCAD DXF File Format Summary
DXF.TXT – Drawing Interchange and File Formats
DXF10.spec – Drawing Interchange and File Formats
Minimum Requirements for Creating a DXF File of a 3D Model
Open Design Specification for .dwg files (PDF)
Structure Of DXF File Format
Chapter 6 — ENTITIES Section – POLYLINE
KLayout – DXF Format
DXF, Revision 12

Are there any libraries for parsing AutoCAD files?

QCAD – What is dxflib?
QCAD – Download dxflib (OpenSource)

Using AutoCAD file formats with open source libraries

  0                # 
SECTION            # Section

  2                #
HEADER             # Header

  9                #
$ACADVER           # Key

  1                #
AC1018             # Value

  9                #
$ACADMAINTVER      # Key

 70                #
     0             # Value

  0                #
ENDSEC             # End Section

[...]

  0                #
SECTION            # Section

  2                #
ENTITIES           # Section Name: "Entities"

  0                #
POLYLINE           # First Entity

  5                #
40                 # Polyline ID

330                #
1F                 # 

100                #
AcDbEntity         #

  8                #
sheet              # Layer Name: "sheet"

[...]

 10                #
121.6670087034155  # Groupcode Start X: 121.6670087034155

 20                #
166.9818181818182  # Groupcode Start Y: 166.9818181818182

 30                #
0.0                # Groupcode Start Z: 0.0

  0
VERTEX             # Add new vertex

  5                #
42                 # Element Reference: 42