Minecraft wiki entry for Setting_up_a_server, associated text: If you are playing on the same machine on which the server is running, select the "Multiplayer" option in the game client, click direct connect, and then type in localhost instead of an IP address.
Both hosting and playing on the same machine is not a recommended practice unless you have a powerful computer (e.g. more than 6 gigabytes of ram (4 for the server, 2 for the client, and some for the rest of the system).
Users within your local network (i.e., that are accessing the same router) can connect using your internal IP address; port forwarding is not required for such local connections. The internal IP address of a specific network adapter can be found by typing "ipconfig" into the command prompt and looking for the IPv4 address, or by using this website. If the port is set to a number other than 25565 in server.properties, that port must be included. This address (both IP and port) will look something like 192.168.0.168:25565.
Users connecting from the Internet (i.e., outside of your local network) must connect using your external IP address. You must port forward for someone outside your network to connect to the server.