You may have noticed that two different SDHC cards of the same capacity have prices that are a little different. Sometimes that is simply the pricing strategies of the manufacturers but sometimes it is because one card is faster than the other at reading/writing data. A faster SDHC card in your digital camera or the MicroSD card in your cell phone allow you to take your next photo more quickly.
Once you take a picture on your digital camera or cell phone, that data has to be written to the memory card. If you have a camera with a high-resolution sensor of , say, 16MB, it will take longer to write the data for that photo than, say, a camera or cell phone with an 8MB sensor. A faster memory card gets that data written faster than a slower card. This become very important with digital cameras with sensor resolutions of 12MB or larger.
SD, SDHC (the HC is for High Capacity) or micro SD card (usually for cell phones) are rated by transfer speed. You can find this by looking at the advertisement, the card, or the packaging for what "Class" the card is. A Class 4 card transfers data at 4MB per second, a Class 6 at 6MB per second, and a Class 10 at 10MB per second. These are ideal speeds with ideal conditions but a Class 10 card is, in practice, going to be a great deal faster at transferring data than a Class 4 card. It is often worth the extra difference in price to buy a faster card.
USB devices such as flash drives and external hard drives currently come in two flavors: USB 2.0 and USB 3.0. Again, the difference is in how fast data can be written to the device or read from the device. The USB 2.0 standard provides transfer rates of up to 480Mbps or just under 1/2 GB per second. The USB 3.0 standard is about ten times faster at 5GB per second. USB 3.0 flash drives and external hard drives are going to cost a bit more but they are going to be much faster. If speed is not an issue than the lower priced USB 2.0 device is just fine. As with the SD cards, the speeds are under ideal conditions. But, relatively speaking, you should see a remarkable difference between a USB 2.0 and USB 3.0 device when transferring data.
This article touches on the basics and perhaps over simplifies a bit. My goal is to help you understand how SD cards and USB devices are rated for speed so that you can make an educated choice and so that you understand the price differences.