You would have often wanted to get software that is available online. Be it a game, text editor, graphic designing software, programming interface, media player, etc. What would you ideally do to get to the link of that software?
Search for the software using a search engine such as Google, Bing, Yahoo and the list goes on. Once you get the relevant results, you opt to visit a website which looks authentic. And you finally arrive at the juncture where you read its description and decide whether to download the software or not. If in case you decide to download the software onto your computer, you click on the download link and the software begins to download.

Once downloaded it sits at the location in your computer which is meant to store all your downloaded files. The downloaded software would come in a compressed format such as .zip or .rar. This means you will have to uncompress files and then begin to use it. So you use a software to extract the files, which get stored at the desired location specified in the extracting software. After extracting the files, you get to see a folder with some more folders and files within it. One of the files within this folder of extracted files would have an extension .exe. Post this you feel, you have the software on your system and it should be ready to be used. Is that it? Can you use the software now on?
No. This isn’t it.

The downloaded software is like a present you just received, packed in a gift box.

The gift is available for use but cannot be used. So what does it take to make an available software usable? You need to install it. That is to say, you need to unpack the gift to use it.

What is software installation?

File Size:

Suppose you downloaded a software of the file size 56 MB. When you download a software, it is condensed.
Coming back to our analogy of a packed gift you received. Let us suppose you received a play station. While it was being packed, its attachments would have been dismantled and packed to occupy less space. Assuring the safety of the product at the same time. When you unpack the play station and begin to assemble all the attachments to make it fully functional, you will observe it occupies more space when compared to the packaged gift.
Similarly, when we install the software we just downloaded, the file size of the installed software increases compared to the one that was downloaded. For the simple reasons, the software was downloaded in the compressed form. After extracting the files, we did not get the software installed. Once installed, it will occupy more space, including the extracted files.

Location:

The downloaded software can reside at any location on the computer whereas, the installed software will always reside on the same system drive where the operating system is installed.
Again, let’s get back to the analogy. You can keep the play station that you received as gift pack anywhere you wish to. But, when it comes to using the gift, you would place and install it at the location where you have the power supply option, proper space and lighting for playing.
Similarly, you would need the software installed on the same drive where the operating system is installed, so that the software upon installation can communicate with the operating system and function as it is supposed to.

Access:

Whenever you would want to use the software, you would go to installed software file, not the downloaded file.
The only difference that exists between our analogy and the software installation is that once you unpack the play station and install it, the packaged gift goes away. Whereas, when you download and subsequently install the software, downloaded file still exists at the location it was downloaded to. Thus, in future, if need be, the software can be uninstalled and installed again using the downloaded file. You do not need to go through the process of searching for the software online and then downloading it over and over again.
It is always advisable to keep the downloaded i.e. the packaged software at a separate location from the operating system. Location here essentially means the system drive. Let us say, you downloaded the software to D: (D Drive), Operating system is already installed on C: (C Drive). After installing the software, the system automatically places it within the C: so that the system files can read it. In such a case, let the downloaded files remain at D: separately. Keeping them separate helps to retain the software in case the operating system gets corrupted for any reason, you can reinstall the software from the downloaded files. Reinstalling an operating system means you would need to reinstall all the software that was earlier present on your system for use.

While using a smart phone or a tablet, you would have often come across this entire process of downloading and installing. This could be when you download an app from google play store in case of android operating system or App store in case of iOS for Apple phones. The file first gets downloaded and then at the time of installation it asks for permissions. This is because at the time of installation is when the software or the App files are extracted and rendered useful on the phone.

Uploading

Do you feel the difference between pull and push? When you go down with something, you are pulling yourself, whereas, when you go up with something you are pushing yourself. And when you pull, you get something and when you push, you give something.
Similar is the case of downloading and uploading. When you download files, you get the files on the system you are working on. Whereas, when you upload a file, you give away the file to the system you are working with. It could be any kind of file, a simple document, media or text file, or entire software. Whatever, you wish to send to the other computer needs to be uploaded to it.
While using a smart phone you would have often shared files with your friends or family using some App. When you send some picture or document from your phone to someone else, you are actually uploading that file to the App server. From the App server it is sent to the specified user, your friend. Hence your friend will be downloading the same file that you uploaded to the server.

Even while using a navigation app, like Google Maps, you are uploading your real time data of your location, according to which the app downloads the relevant route data on the map for you to reach the specified destination.