I am new to this forum and I don't know much about PCs and OS. All I want to do, if possible, is to install elementary OS on my notebook without erasing Windows 10. I would also like to know that after installing elementary OS, is it possible to boot in this OS without a bootable USB drive.
You can install the elementary OS on a separate partition or over free space on the hard drive. You will only need an external storage device to install the OS.
I highly recommend you learn a lot more about PC and OS's before you install a Linux distro, but here's a quick premier:
A PC is the actual physical laptop/desktop
An OS is the software layer that coordinates all your hardware in order to run applications (like Word, or Google Chrome)
Windows 7/8/10 is an OS, Elementary OS is also an OS
Elementary OS is based on the Ubuntu OS
Now as to install Windows and Elementary OS at the sametime search up Dual Booting and you'll find tutorials on how to install Windows and Elementary OS at the same time, on the same hard drive (or ssd). You'll need a bootable USB Drive or CD/DVD Drive in order to install Elementary OS alongside Windows, but you do not need it in order to boot into Elementary OS
As for using a USB drive to boot into elementary OS, you don't need one. You can also use a DVD (not sure if a CD is big enough) and burn the elementary OS .iso to one (using ImgBurn or something similar). However, it will be slower than using a USB drive.