We have a FAQ at:
http://www.nfrpartners.com/cncfaq.htm
that includes some info on basic G-codes. Also, following are 2 programs that create the same design, which is a very simple bird-like shape that we use for testing. The first version uses absolute (G90) coordinates and the second uses incremental (G91) moves.
% Bird
(ABSOLUTE - G90)
N10 G20 G40 G80 G90
N20 T1 M6
N30 S3000 M3
N40 G1 X-1.2509 Y1.25 Z0. F15.
N50 G2 X-.2607 Y1.1429 I.5009 J0.
N60 G1 X-.4811 Y.136
N70 G3 X.4465 Y-.2251 I.4811 J-.136
N80 G1 X1.3035 Y1.4751
N90 G2 X2.1718 Y.9816 I.4465 J-.2251
N100 G1 X.4218 Y-1.7684
N110 G2 X-.4824 Y-1.6316 I-.4218 J.2684
N120 G1 X-1.2324 Y1.1184
N130 G0 Z5.
N140 X-1.5 Y1.25
N150 M5
N160 M30
%
% Bird
(INCREMENTAL - G91)
N10 G20 G40 G80 G91
N20 T1 M6
N30 S3000 M3
N40 G1 X-1.2509 Y1.2500 Z.0000 F15.
N50 G2 X.9902 Y-.1071 I.5009 J.0000
N60 G1 X-.2204 Y-1.0070
N70 G3 X.9276 Y-.3611 I.4811 J-.1360
N80 G1 X.8570 Y1.7002
N90 G2 X.8683 Y-.4935 I.4465 J-.2251
N100 G1 X-1.7500 Y-2.7500
N110 G2 X-.9042 Y.1368 I-.4218 J.2684
N120 G1 X-.7500 Y2.7500
N130 X-.3000 Y.1300
N140 M5
N150 M30
%
I hope it's useful.