All IGPs are GPUs. Not all GPUs are IGPs.
- GPU is a Graphics Processing Unit.
- IGP is an Integrated Graphics Processor.
Integrated means that the chip is physically part of the motherboard, and typically uses part of the system RAM ("shared memory") rather than its own dedicated memory. A non-integrated GPU is usually part of an expansion card -- PCI(e) for desktops, or Mini-PCI(e) for laptops -- and has its own dedicated graphics memory on the card.
Most laptops use a single GPU is to drive both the built-in LCD and any external video outputs (VGA/DVI/HDMI/etc).
The "waviness" you see on your VGA monitor is more likely due to your graphics settings. Try changing output resolutions or refresh rates (or both) to see if you can get a better picture. It could be an indication that your GPU is underpowered for running two displays, or it could be an indication that your VGA port is damaged.
According to ThinkWiki, the T60 uses either an Intel GMA 950 (integrated) or an ATI Mobility Radeon x1300 or x1400 (PCIe cards).