I want to ask about the transfer speed of the memory card. It is written on the memory card as something like 45MB/s, 80MB/s. Does it affect anything when we take pictures? What I understand is that the higher number we use, the more rapidly we can take pictures, so it is good for continuous shooting. Is that right? Is there anything else?
Most cameras have an internal buffer they write the pictures to first. From there, the images are written out to the (much slower) memory card. So, if you take pictures at maximum speed, your buffer will be full after some time (My Canon EOS 60D can make about 50 JPEGs or 12 RAW images until the buffer is full).
After that, I have to wait until at least some images have been written out to the memory card. And that process will be faster if the write speed of the card is faster.
Also note that some cameras cannot take advantage of the highest speeds of the cards. My 60D e.g. cannot use the highest speeds, so I stick to class 10 cards (meaning 10MB/s write speed)
So, continuous shooting is not possible, and the breaks between the bursts will be shorter the faster your card (and camera) is.
There are two speeds and it is critical that you separate them:
The read speed affects how fast images can be transferred from the memory card. It is mostly relevant to get images into your computer and a little for reviewing images. It has no effect on shooting speed.
The write speed is how fast can data be stored on the card. This is the limiting factor when the camera saves images from its internal buffer.
Make sure you are looking at the write speed to determine who fast a card is for shooting. Still, there are bottlenecks on along the path which means that you will rarely get out the maximum cards-speed for reading or writing:
The continuous drive is limited by mechanics, electronics and processing, so a faster card will almost never improve the maximum shooting rate. What is does improve is the sustained burst length. As the card is faster, a camera can write to it and free space from its internal buffer which leaves more room for the next shot. That's why you can shoot longer. In some cases you can keeping doing so until the card capacity fills up.
The read-speed is essentially limited by the device doing the reading. If you connect your camera to the computer, usually speeds are quite slow and very few devices can transfer at more than USB 2.0 speeds. Some support USB 3.0. Ideally though, you would use a card-reader and that also has a speed limit but you can easily find USB 3.0 ones.