If you, like me, use Chrome then at the top right hand corner of the bar there is a little square with three horizontal lines. If you click on that you get a drop down menu where you can select incognito browsing or you can press Ctrl + Shift + N and get a window where your browsing history isn't tracked. You get a cute pic in the corner that looks like this
Incognito browsing is apparently sold as 'you can shop for presents and keep them a surprise from your other half.' In the real world it is frequently used for looking at pictures and videos of 'adult' content. I have learned to only click on stuff from Mumsnet in an incognito window, as bear sometimes uses my laptop and there is no shame on that site.
However, if you are trying to browse for something that you may end up buying online or you are trying to work out a good price or a better deal, it is worth using incognito browsing. If you go on a cashback site, it checks to see if you have already been on the site by looking at cookies on your machine. Usually you can only use a cashback site if you allow cookies. If you have browsed, for example, M&S directly and then try and go through the cashback site, it could be refused. I suppose the reasoning is, why should they give you cashback when you were going there anyway? I don't want to risk getting refused a big amount of cashback because I have clicked through an advert or a link that I didn't realise went through to a different site. Incognito browsing doesn't allow cookies.
If you do find that you have accidentally gone through direct to a site where you will be making a purchase that will get you a lot of cashback, you can go back and clear cookies. You go back to the same little three horizontal lines, go down until you find 'settings' then go down again until you find 'advance settings' and then you can find 'clear browsing data'. This will log you out of everything and you may have to re-enter every password again, but if you are getting a 3% deal on a £500 appliance then it is worth it. Perhaps not so much so for 1% on a £3 purchase.
As a note, a little warning comes up to say that this doesn't hide your history from your employer or internet provider. A site you are on can tell that you are on it. I am confident that it is not a way to hide illegal activity. As I have found arsenic on ebay, and searched the best place to find slow match for explosives for Digging up the Past, I think I'll just worry about shopping cookies.