It seems that most hardware synths use subtractive synthesis techniques, while in the digital realm it's common to see synths that work with different degrees of additive synthesis.
What's the difference between these two techniques?
Additive synthesis describes the process of combining (adding) sounds together to make new sounds, hence the term "additive". "add" ... you add stuff together.
Subtractive synthesis describes the process of making new sounds by removing elements of a sound through processing such as filters, hence the term "subtractive". "subtract" - you remove stuff.
Both are equally valid techniques that can be used effectively in combination.
Here's an interesting video that explains additive synthesis in a bit more detail: