These normally come with the pool with plastic being preferable to galvanised steel. You can also get underwater steps for pools sunk in the ground and have a deck surround. They look like wedding cake tiers and suit people that don't want an "ugly" plastic ladder hanging over the edge of the pool.
These are generally bolted at the top coping, the bottom rests inside the pool wall with rubber suction cups to prevent damaging the inside of the pool.
These are completely bolted onto the top coping and overhang the top of the pool. They are best suited for step areas of the pool.
Rails are normally used for the disabled or elderly. They are normally installed near steps for assistance.
Some points to keep in mind are:
They should be 316 marine grade stainless steel (including any nuts and bolts).
What do you want to achieve. Is it to step out of the pool with a full ladder, or just for support on a step area?