Retaining walls Will come in a Varity of shapes, colors, and materials. So how do you know what the right retaining wall for you? By definition a retaining wall supports soil. Depending on the amount of soil will determine the wall you will need. lets start will the understanding of traditional concrete blocks.
Decorative Garden beds under 2' can use a simple Lowes or Home Depot Concrete block.
Walls under 4' A hallow fill concrete block will want to be used. This block must be filled both behind and inside with a drainage aggerate to prevent collapse.
For walls over 4' An Engineer will evaluate the dig and Hight. Also an structurally supporting walls such as walls near a house, barn, pool, road, Ect will require a pin locking block.
Natural Stone, and landscaping timbers
All of these can and will make beautiful strong retaining walls if done correctly. However keep in mind the life expectancy on timbers is 30 years here in Colorado. Much much less in wetter environments. Natural stone can include but not limited to. Boulders, Flagstone, Fieldstone, and quarry cut materials.
Hardscaping Should last a century. Concrete cracks with thaw and frost cycles. also with concrete drying to fast. no one wants those unsightly cracks. Your not getting rid of them. Its no surprise why pavers and flagstone cost more. Take a look at the Roman roads. They might not be beautiful however still completely functional today.