TeamViewer is a Desktop Sharing, Remote Control and Support software described by its author as
TeamViewer establishes connections to any PC all around the world within just a few seconds. You can remote control your partner’s PC as if you were sitting right in front of it.
TeamViewer makes it very easy to connect to a remote computer for remote control, file sharing or just making presentations. It is even possible to control unattended computers if the software is used as a service. It easily bypasses routers without the need for port-forwarding or other unnecessary and perhaps complex tweaks. 
After installing and running the full version the window seen here opens which is ready to give access to the host computer or to connect to a remote computer once the ID and password of the said computer is received. A host or client computer may also download and run without installation a ‘quick support’ version making it very handy for quick help or just help to the less experienced computer user.
Security should be very important here because pretty broad access is given to computers using TeamViewer. Here encryption is based on RSA private/public key exchange and AES (256 Bit) session encoding. Plus the password is re-generated for each time the program is used adding another layer of protection.
TeamViewer is free for private/non-commercial use is only 1.8MB for the full version.