So to data have to be used with care and a good comparison shall use as many genes as possible. I will use music as an example to understand the concepts. This will allow to define the concept of genetic distance. Genetic distance between two populations is just how different the corresponding songs are. Because a song is made up of many notes it could happen that some parts of the tune sounds rather similar but the overall composition is still very different.
Genetic distance is expressed as a value between pair of populations.
More or less in the same way that it is possible to define distances between any pair of points on the Earth, or it will be possible the define the degree of difference between songs by adding the differences note after note. To be able to put a little bit of order on the chaos the second concept use is called Principal Components. They can be understood as the different musical instruments that can by used to play the different songs. Each instrument will have its own harmonies that will fit better or worse in the interpretation of the different songs. In principle each song can be described as a combination of just a few well chosen musical instruments.