Very simply explained, it is your Home or Business internet connection.
Also known as your Home or Business Broadband.
It uses your existing telephone line into your house or office.
ADSL is a type of Broadband Internet connection.
Before jumping into a more detailed ADSL explanation, lets explain DSL first.
Or "Digital Subscriber Line" (DSL).
Very simply put, it is your connection to the internet. From your computer to the world wide web.
There are lots of different names for the internet: broadband, ADSL, NBN, cable, wireless, mobile broadband, 3G, 4G, LTE, etc.
Internet connections can be broken into categories:
Broadband as opposed to the old "Dial-up" is just a technology to describe the way you connect to the internet.
There are a few types of broadband, like NBN, DSL, ADSL, SDSL
ADSL can be differentiated by the fixed copper connection it needs between your Exchange and your home or office.
ADSL has the "A" in front of "DSL" because it's "Asynchronous" - meaning exactly that - "not" going at the same rate. There is an up and a down part to each line or internet connection. In ADSL, the down part of the line is different to the up part. Think of an email, if you receive an email it is the down part, as in download. Sending an email is the "up" part or upload, it's going up to the magical internet clouds.
For the average user, they will be viewing or receiving or downloading much more than they upload. Generally a lot more websites are read and viewed, meaning the content is "downloaded" as opposed to content that is being uploaded. For example, a typical site where you would look around to find what you need, and depending on the site, perhaps you may buy something or fill in your details or fill in a contact form. That's pretty much the only "up" part of the whole experience you will contribute...otherwise it is images and content from every single page you view and you are pulling all this info "down" to view on your computer.
So in most cases the down pipe needs to be bigger than the up pipe.
Well, it doesn't need to be different, it's just that with existing infrastructure (your copper phone line to your house or office), it makes sense to maximise its use. It was a good idea to provide more "lanes" on the one side of the road than on the other side that wasn't being used as much.
And even for the very few that do a lot more uploading, like web designers/developers who put the content onto the internet...they generally would still need a bigger download pipe because they're also like the rest of us, they also do lots of downloading and general browsing of the internet.
NBN™ (National Broadband Network)
The National Broadband Network (NBN) is still a broadband connection and also a fixed connection like ADSL, but in this case, it's not copper, it is fibre. This means it can travel at the speed of light which means it is much faster than copper. However with lots and lots of factors to consider and financial implications, it is not as simple as this and the NBN connection to your premises may in certain cases be made up of various technologies.
Another thing to note with these various technologies, is that it is not always limited to a fixed connection and it is possible to also get Wireless NBN in certain areas, which is a mixture of fixed fibre, up until a point (that it is financially viable to install) and then the remaining connection to the end user's premises may be wireless.
You can get a Synchronous line *SDSL" where the lanes are equal. For some businesses, uploading is as important as downloading, because they are putting lots of content up on the internet for all of us to see (or perhaps for another branch of theirs to access it). Sometimes time is a constraint, and therefore a bigger "up" link will help.
WiFi is wireless right? Yes it is, but on it's own, it doesn't have an internet connection, it is just the last little bit of the whole "internet thing", so your computer doesn't need wires.
The source of the WiFi (or the actual Access Point) still needs ADSL or some kind of main internet connection to be able to pass on the internet connection wirelessly.
Think of the average modem/router in a house. It has the telephone line connecting into the modem and then you could plug an ethernet cable (generally the blue one that looks like a bigger phone line) into your computer. Or, you could setup the WiFi part of your router and on your computer enter in the password...and voila, you're connected wirelessly to your internet connection (which still has the telephone cable from your modem going out your house). If you unplugged the telephone line you will see you are still "connected" wirelessly to the WiFi network, however your internet won't work anymore!
Proper WiFi would be a big feed coming into a central location and then that signal is wirelessly transmitted over quite a big distance. You may need a receiver, or modem in your house to receive the wireless connection and then you could even have a cord from your computer to that modem. But hang-on that's doesn't sound very wireless to me?
The main connection is wireless, i.e. no copper running into your house. In other words no need for a telephone. Once the signal is in your house, you can connect to it as usual. With or without wires...it's still wireless.
The NBN will start making use of a mix of this technology to get to all the locations they need to.
Mobile broadband is wireless too...isn't it? Well yes, there are no wires, but technically...no!
Mobile broadband is mobile broadband. Your mobile broadband device thingy, or your mobile phone, act as the modem, no wires, but you need a SIM card and you need data on the card. The main difference is wireless would generally be free, because the internet connection is already provided, accounted for and being paid for, and the last "wireless" part to your computer is free.
Mobile broadband is still quite expensive relatively and you pay for all the data usage. That's why most of the clever phones these days will automatically search and hook you up to an available WiFi network to save your mobile broadband usage and costs.