Guitar basics: Hammer-ons
You can change between two notes by picking them separately, but if the second note is higher than the first in pitch, you can play what's called a hammer-on.
A hammer-on will make the transition between two notes smoother, which will make your playing sound more fluent.
This video lesson shows you how to perform hammer-ons correctly and provides you with two examples to try for yourself. These examples come with free tab.