Upload is also known as uplink and means the one way connection from a device (phone, PDA, laptop computer) to a network or server (mobile phone network, internet etc.)
While downloading is receiving a file from another computer, uploading is the exact opposite. It is sending a file from your computer to another system. Pretty straight forward. It is possible to upload and download at the same time, but it may cause slower transfer speeds, especially if you have a low bandwidth connection. Because most files are located on Internet servers, people generally do a lot more downloading than uploading.