Quite some time when there is a need to explain someone how do perform a certain activity on the computer, people spend a lot of time on the phone saying go there and click this and so on. Some of the support companies use remote desktop software like VNC and PC Anywhere which are not free and require additional setups on the router, port forwarding and most of the time to have an external IP address or be in the same network.
For the daily use when I try to help my relatives and friends with some tasks, I recommend TeamViewer application that is extremely easy to use, doesn't require setups and is based on P2P network, meaning you will be able to connect to another computer even if you are behind the firewall or a router that doesn't allow forwarding ports.
The person who is the host will run a small application and it will generate an ID and a Password and then the remote person will get the details over the messenger or phone and will enter the same details to connect to that computer.
This application is free for the home users (non-commercial).