Flatbed scanners are the most common type of scanner. The photos are placed on a glass scanning bed where they are then converted into a digital image on the computer, and then stored to the computer hard drive.
There are non-flatbed scanners that come with what is called an ADF or automatic document feeder that will rapidly scan a stack of photos. Scanning a multipage document into a PDF (Portable Document File) is best accomplished with this type of scanner. Scanners may be designed ONLY to rapidly scan photos and NOT paper, so be sure you are aware of the scanner characteristics. These scanners can scan up to 50 images/minute and a little slower if scanning both sides.
There are many different types of slide scanners. Some use a camera attached to a Kodak Carousel to scan each slide. Others may use a long slide tray. One of the newer slide scanners made my Kodak is the Scanza (image below) and can also quickly scan and produce good quality scans, although you manually (thought quickly) push the slides through a small tray. There are some flatbed scanners that can also scan slides.
A standard HD (high definition) computer monitor has 1,920 pixels across and 1,084 pixels in height. Standard digital TVs are now "4K" and higher are 3,840 pixels across and 2,160 pixels in height. An Apple iMac 5K screen has 5,120 pixels across and 2,880 pixels in height. Why is this information important? If you are going to display your photos on a television screen, you need to know at what resolution you will scan your photos at.
For most people just preserving photos to use in a book, or to share with others, the standard "HD" resolutions of 1,920 PPI across X 1,084 PPI in height should be sufficient to give you good printing quality.
3" wide X 2" tall Photo
1920 ÷ 3 (width of photo) = 640
Select a value of 600 DPI to scan at.
6" wide X 4" tall Photo
1920 ÷ 6 (width of photo) = 480
Select a value of ~ 450 DPI to scan at.
8" wide X 10" tall Photo
1920 ÷ 8 (width of photo) = 240
Select a value of 300 DPI to scan at.
1.5" wide X 1" tall SLIDES
1920 ÷ 1.5 (width of slide) = 1200
Select a value of at least 1200, but in practice, I find it better to scan at higher values such as 3200 DPI.