All steroids are both. You need both in order for them to work, but some have characteristics that lean more one way or the other.
In other words, testosterone is pure androgen. It does what androgens do -- increased strength, libido, muscle hardness. It also fills the muscles with water making them bigger and it increases appetite. It also retains nitrogen, which is what makes muscles grow.
Primobolan is on the other end of the spectrum. It's only slightly androgenic but its nitrogen retaining qualities are high. Since superphysiological dosages of androgen are what cause many of the negative side effects it's preferable to get as much of the "anabolic" factor as possible. Unfortunately, the results are slow and people want fast results, even if they're more temporary.