VGA is analog and DVI is digital.
What it means is that some graphic features are not supported on VGA. features like Nvidia 3D vision and DSR*
Some resolution with specific refresh rate** are not supported ether. Like Full HD at 120hz.
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* DSR (Dynamic Super Resolution) is a feature for games that renders the game in a higher resolution than what's supported on the monitor and then downscales it to match the monitor.
** Refresh rate is the number of times the monitor refreshes the image in a second.
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But more important for you is the fact that the VGA signal quality gets worse and worse the longer the cable is.
For VGA cables the quality actually matters. Not so much with cables like HDMI, but a quality VGA can actually increase image quality. (If a sells man ever try to sell you a more expensive HDMI cable because he says it gives you a better image, punch him. He is lying)
Good VGA cables helps reduce the loss in quality. But it's something you have to buy besides the monitor. You see, all included VGA cables are of poor quality. So you have to spend extra money just to get the image quality that you paid for when getting a new monitor.
And long VGA cables pretty much HAVE to be of a certain quality if you don't want a noticeable reduction in image quality.
Digital cables (like DVI) is better in the way that, ether there is a signal with 100% of the quality or no signal at all.
