This is a very comprehensive, pretty clear introduction to grid-based website design. Vinh's arguments for why we should use grids are easy to agree with, and his description of the different nature of web grids compared to print grids is insightful.
The main takeaway here is that grids are the right tool to use, because they're clear and everything just kinda works that way (both for us as readers and for the technical pieces that make up the web) and that in order to create good designs for the web one must understand the contraints. Vinh outlines some constraints that are universal to web design, like the wide variety of different viewport sizes and rendering software that one's page must be ready to interact with. Then there are project-specific constraints, like the business needs or the specific purpose of each page.
One point I found particularly interesting was what he said about how defining more constraints can counterintuitively make the design project easier. On reflection, this is certainly something I've encountered with both design and coding assignments I've had in different classes. The more specific and plentiful the requirements are, the fewer wide-open choices there are to make. And keeping with Vinh's ideas, the more you can frame the requirements and desires for a site's function into specific constraints on your process, perhaps the easier it is to then make decisions about how to set up your pages.