Some cartridges are available in more than one "capacity" or "yield". This refers to the amount of ink in the cartridge, and by extension, the number of prints that you can expect it to produce.
Standard yield cartridges are the most basic option. They have the lowest dollar price tag, and the least ink in them, but the cost per page is usually more expensive than the higher yield options. Standard yield cartridges are the best option for someone prints little and rarely.
High yield cartridges (often denoted by "XL" after the number) have more ink than standard yield. They are of course more expensive, but the cost per page is often considerably less. This makes them cheaper in the long run for users who print regularly.
Extra high yield cartridges are not available in every range, but as the name suggests, they contain even more ink than the high yield, and are usually cheaper per page to print.
The standard yield HP61 above (blue packaging) is rated at 190 pages while the high yield HP61XL (green packaging) is advertised as printing "up to 2.5x more" because it's rated at 480 pages. For this type of cartridge, the high yield version is typically a little less than twice the price of the standard yield, which means that the cost of printing each page is lower.
For example, if the HP61 costs $25, each page would cost about 13c to print, while if the HP61XL costs $45, each page would cost just over 9c to print.
Every manufacturer has a different way of identifying the standard, high and extra high yield cartridges. Sometimes it's as simple as a suffix at the end of the part number, but sometimes the number itself is different. It gets more complicated when manufacturers use more than one part number for the same cartridge (this usually happens when they develop a new cartridge and assign it a number in their own internal sequence, but want something easier to remember when it comes to selling it - for example the HP61 is a CH561WN and the HP61XL is a CH563WN).
Higher yield cartridges are often the same size as the standard versions, but there are exceptions. The Epson 127 (the high yield version of the 126) is twice as wide, which can come as a surprise if you've never seen one before, because it looks as though it's too big for the printer.
However, if you look closely, you'll see that the clip and chip are on one side of the cartridge - it's just like a standard single width version with an extra bit stuck on the side.
This isn't uncommon, especially with compatible versions, where manufacturers will use every bit of space available to make the cartridge the highest possible capacity.
WHICH CARTRIDGES WILL FIT IN MY PRINTER?
Your printer manual will tell you which cartridges are suitable, but if you don't have that to hand (and who does?) you can just visit our website, type your printer model number in the search bar at the top and go to the page specific to your printer. On printer specific pages, we list ONLY the cartridges that will fit, and nothing else, so you can be sure you're getting the right ones.