Some source control tools require a server and a client in order to be operational. Others only require a client. Still others work perfectly with a client that works as a local server as well. Please read the article on requirements gathering before deciding.
In short: all depends on your set-up.
Follow here for client installations.
Follow here for server installations.
The following tools require you to set-up a server and a client:
The following tools require you to set-up a server and a client but can be hosted on the same machine:
The following tools can connect to a server but also work stand-alone:
The following tools work only as a stand-alone client:
- none listed