In computer networks, to download means to receive data to a local system from a remote system, or to initiate such a data transfer. Examples of a remote system from which a download might be performed include a webserver, FTP server, email server, or other similar systems. Downloading or uploading is relative to the system involved, for example a PC downloads a file from a server while the server is uploading that file to that PC, it has nothing to do with the size of the systems involved (see Sideload below).
A download can mean either any file that is offered for downloading or that has been downloaded, or the process of receiving such a file.
It has become more common to mistake and confuse the meaning of downloading and installing or simply combine them incorrectly together.
The inverse operation, uploading, can refer to the sending of data from a local system to a remote system such as a server or another client with the intent that the remote system should store a copy of the data being transferred, or the initiation of such a process. The words first came into popular usage among computer users with the increased popularity of Bulletin Board Systems (BBSs), facilitated by the widespread distribution and implementation of dial-up access the in the 1970s.
The use of the terms uploading and downloading often imply that the data sent or received is to be stored permanently, or at least stored more than temporarily. In contrast, the term downloading is distinguished from the related concept of streaming, which indicates the receiving of data that is used near immediately as it is received, while the transmission is still in progress and which may not be stored long-term, whereas in a process described using the term downloading, this would imply that the data is only usable when it has been received in its entirety. Increasingly, websites that offer streaming media or media displayed in-browser, such as YouTube, and which place restrictions on the ability of users to save these materials to their computers after they have been received, say that downloading is not permitted.[1] In this context, "download" implies specifically "receive and save" instead of simply "receive". However, it is also important to note that "downloading" is not the same as "transferring" (i.e., sending/receiving data between two storage devices would be a transferral of data, but receiving data from the Internet would be considered a download of data)
(source : http://en.wikipedia.org/wiki/Uploading_and_downloading )
