When measuring internet speed, the measurement can be broken down into three categories, each with their own separate purpose and function. These categories are: download speed, upload speed and latency.
Download speed refers to the speed at which a user receives information from a location on the internet, such as streaming videos on YouTube. On the other hand, upload speed measures the speed at which as user can send information to a location on the internet, such as attaching files or photos to an email. Latency is a little bit more removed from download or upload, instead it measures the length of time that occurs between the transfer of information.
Usually when you choose an internet service provider (ISP), you will be quoted a download speed and upload speed, for example your plan might be 50 Mbps download speed and 2 Mbps upload speed. Think of these numbers as the amount of information that can be moved by your internet connection, the higher the number on your download speed, the less time it should take you to receive large files. Usually upload speeds are much lower because we spend more time receiving large files than we do sending them out ourselves. Remember that the figures quoted to you by your service provider measure the maximum internet speed you will receive, and on average you may be running at a slower speed than that.
Latency is usually less relevant to average consumers using the internet for everyday purposes, it is measured by determining the time taken for information to travel to a source and back. On DSL or cable internet connections, a latency of less than 100 milliseconds (ms) is considered to be typical, whilst 25 ms is the desired ideal.