Grid based design is taking a look at the mockup of the webpage you would like to create and dividing it up into grids that fit that design. Generally each grid will take up a proportion of the page divisible by 12 eg.(3/12, 4/12, 6/12, 12/12)
Responsive webpages make it easier to view the page on any type of device, regardless of size.
Semantic structure is tagging elements of the page to label them more clearly and make it easier to make chenges to certain elements individually.
I think bootstrap will be very interesting to play around with, and combined with google fonts it should be a lot of fun to try out creating sites in spare time.