When the bootloader passes control to the kernel it has to pass some basic information to the kernel, which may include some of the following:
The kernel command line is a plain ASCII string which controls the behavior of Linux, setting, for example, the device that contains the root filesystem. I will look at the details of this in the next chapter. It is common to provide the root filesystem as a RAM disk, in which case it is the responsibility of the bootloader to load the RAM disk image into memory. I will cover the way you create initial RAM disks in Chapter 5, Building a Root Filesystem.
The way this information is passed is dependent on the architecture and has changed in recent years. For instance, with PowerPC, the bootloader simply used to pass a pointer to a board information structure, whereas, with ARM, it passed a pointer to a list of "A tags". There is a good description of the format of the kernel source in Documentation/arm/Booting.
In both cases, the amount of information passed was very limited, leaving the bulk of it to be discovered at runtime or hard-coded into the kernel as "platform data". The widespread use of platform data meant that each device had to have a kernel configured and modified for that platform. A better way was needed, and that way is the device tree. In the ARM world, the move away from A tags began in earnest in February 2013 with the release of Linux 3.8 but there are still quite a lot of devices in the field, and even in development, still using A tags.