π, κ, and τ are "unvoiced" because the voice box is not used in their pronunciation.
β, γ, and δ are "voiced" because the voice box is used. (Place your fingers on your voice box and pronounce these letters. You will feel it vibrate when you say the voiced stops.)
The final column of stops, φ, χ, and θ, technically are not stops but "aspirates" because the air flow is only slowed down. However, because they fit into the pattern, it is easier to view them as stops. The rough breathing is also an aspirate.