Over the years, I've accumulated a bunch of Ethernet cables. I've kept a stash of cables of different lengths in a box - you'd be surprised how handy a 10 metre cable can be - as well as some short ones of just 25 centimetres and various in-between lengths. Here's how to tell what ethernet cables you're using and why it matters.
You'd think that all Ethernet cables are basically the same but they're not. There's quite a bit of difference between Cat5, Cat5e and Cat6 cables once you peal back the protective sheath. When different cable types are mixed on your network, your network will perform at the speed of the slowest link.
So, how do you tell what sort of cables you have?
Most ethernet cables have some information printed along the sheath. You're looking for something that says "Category" followed by a number. On some cables, the word "category" is abbreviated to "cat". If the cable doesn't have anything printed on it, you could use a cable testing device but I suspect that it's probably not worth the effort. But the main thing is that Cat 5, 5e and 6 will be good enough for most of us and anything else is hard to find.
If you're planning to cable your home, I'd advise that you ensure everything is being done with Category 6 cables and that you connect your wired devices using Category 6 as well. This will ensure you're getting the fastest possible connections. It's important to note that even with cabling inside your home or office that is capable of gigabit or better speeds, that your internet speed is governed by what your ISP or RSP delivers. But transferring files between computers, your NAS and internal streaming devices will be enhanced by faster cabling.
It's worth noting that while some vendors are selling Cat 7 cables, this isn't an officially recognised standard. There is a Category 8 that is offical but it's intended for specific uses within data centres and not general purpose network cabling.
If you have a mix of cables today - and I suspect most of us have that - don't worry too much. Over the relatively short distances we're talking about you're not likely to see massive performance differences. But if you a stickler for having everything uniform you can change cables over easily and cheaply.