G-code consists of a series of alphanumeric codes that tell the CNC machine how to move, position, and control tools to create a part.

Each G-code corresponds to a specific function or operation. 

Here are some common G-code commands:

G00 - Rapid Positioning: Moves the tool rapidly to a specified position without cutting.

Example:

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G00 X1.0 Y2.0 Z3.0
G01 - Linear Interpolation: Moves the tool in a straight line to a specified position at a defined feed rate.

Example:

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G01 X1.0 Y2.0 Z3.0 F100
G02/G03 - Circular Interpolation: Moves the tool in a clockwise (G02) or counterclockwise (G03) arc to a specified position.

Example:

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G02 X2.0 Y2.0 I1.0 J0.0
G90 - Absolute Positioning: Sets the machine to interpret coordinates as absolute positions.

Example:

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G90
G91 - Incremental Positioning: Sets the machine to interpret coordinates as incremental, or relative to the current position.

Example:

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G91
M03/M04 - Spindle On Clockwise/Counterclockwise: Turns on the spindle in the clockwise (M03) or counterclockwise (M04) direction.

Example:

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M03 S1000
M05 - Spindle Off: Turns off the spindle.

Example:

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M05
M08/M09 - Coolant On/Off: Turns on (M08) or off (M09) the coolant.

Example:

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M08
G28 - Home Position: Moves the tool to the home position.

Example:

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G28
G53 - Machine Coordinate System: Commands in the machine coordinate system.

Example:

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G53 G00 X0 Y0 Z0

It's important to note that the specific G-code commands and their syntax may vary depending on the CNC machine and its controller. Always refer to the machine's documentation for accurate and machine-specific information. Additionally, CAM (Computer-Aided Manufacturing) software is often used to generate G-code based on a 3D model or a design, making the programming process more user-friendly.