There are 4 main types of advertising, CPC, CPM, Sponsored comparisons and media buying.
CPC or Cost Per Click advertising is when you display an ad and get charged whenever it is clicked on. The most common place to do this is through Google Adwords. Adwords shows your ads at the top of google search results when a relevant search is made, but usually not when that search can be better served by shopping results, when shopping results are more relevant, you'll usually see Sponsored comparison type ads instead.
The Lucozade energy link at the top of this result is a typical example of a CPC ad via Google Adwords
Sponsored comparison is a form of advertising in which you pay to have your products put into a shopping database and then shown to relevant customers. There are free versions of these websites, however they're rarely used, especially in our market. The most popular paid version is Google Shopping. It works in much the same way as Adwords, however your links are shown with a picture and a price and are usually the ads that show when a search related to possible online shopping is made. 9 times out of 10 you will also see an Adwords link below the shopping ads, but on shopping results pages text ads are rarely clicked.
The 4 images across the top are sponsored comparison ads via Google Shopping.
The wholesale T-shirts link underneath is an Adwords CPC ad.
CPM refers to cost per mille (or cost per thousand). It works almost exactly the same as CPC with the only big difference being that you pay when your ad has been seen 1000 times, regardless of how many times it has been clicked. It's always better to begin a campaign using CPC, but most advertising platforms will let you switch to CPM during a campaign if you want to. This is a good thing to do in certain cases. The keyword 'Flag T-shirt' may cost you £0.30 per click on a CPC model, however if your ad is clicked on more than once every thousand times it is viewed and the CPM is £0.30 or less, it may make sense to switch to CPM there instead. Your also likely to come across cost per mille advertising if you branch out into offline advertising (such as TV, radio, publications, etc.) though this can also sometimes be media buying if the ad is priced on time rather than views.
An advertising budget of £50 with a CPM of £2.50 means that your ad will be seen 20,000 times.
At a profit of £7 per sale, you'd only need a conversion rate of around 0.035% to make your ad spend back.
Media buying is very similar to CPM advertising, however rather than paying for your ad based on impressions (views), you instead pay a set price for a certain amount of time. It is essentially the short-term rental of ad space. Larger websites usually run media buying type ads, but media buying most often relates to platforms such as billboards, bus signs, and those sorts of places. It's very common for media buying opportunities to be overlooked, but in certain cases it's exactly the right way to advertise. For example, if you had made a t-shirt about the Mayweather vs Pacquiao fight, a good place to advertise would have been on sports websites like ESPN for the month leading up to the fight. You can find great media buying opportunities at BuySellAds. Even checking that site for potential spots can help you come up with design ideas.
A screenshot of the gaming section of BuySellAds. A great place to find media buying opportunities for T-shirts about gaming.