BGP Route Server is used at the Internet Exchange Point to simplify the BGP Peering process. Instead of managing, and maintaining hundreds of Peering sessions in a large Internet Exchange Point, BGP Route Server is used.
Every BGP-speaking router has a BGP session with BGP Route Server.
Route Server doesn't change the BGP Attributes, although the type of BGP Peering session is EBGP (Similar to Route Reflector in IBGP).
Figure - BGP Route Server
In the above picture, Redline depicts data plane connections and the green line depicts control plane connections.
Route Servers are usually not deployed as Inline devices. This means they are most of the time deployed purely as control plane nodes.
Traffic doesn't go through BGP Route Servers.
From this aspect, BGP Route Servers can be compared with BGP Route Reflectors.
BGP Route Reflector vs BGP Route Server
Although this is not exactly an apple to apple comparison, most Network Engineers ask this question. Are they similar? Actually, BGP RR and BGP Route Servers have many things in common. Both BGP Route Reflectors and BGP Route Servers are used for Network scalability. BGP RR is used in IBGP and BGP Route Server is used in EBGP. Both are usually deployed as Offline nodes, meaning, traffic doesn't go through them, they are used as commonly control plane nodes.
BGP RR and BGP Route Servers don't change the next hop to themselves. Thus they are not used mainly as data plane nodes. Although in theory, they can, and especially for BGP RR, we can say, many people, to reduce the cost of having an extra node, use one of their internal routers as BGP RR, so they use Inline RR deployment.
BGP Route Server doesn't change the next-hop to itself
Thus it is used only as a Control Plane device, not a Data Plane. This means actual traffic is passed between the companies that participate in the Public Peering Internet Exchange Point and traffic doesn't go through the Route Server.
It is very similar to BGP Router Reflector which is used in IBGP topologies. The difference is, BGP Route Server is used in EBGP.
There are many Open Source BGP Route Server software. Bird, and ExaBGP are the mostly deployed ones by the Internet Exchange Points. From here you can find the comparison of these three routing daemons. Of course, Cisco supports BGP Route Server as well, and many Cisco routers can be used as Route Servers.