questions about 2.4.18

I am preparing to upgrade my kernel. A friend of mine who does support
for BRU Linux said that 2.4.18 was unstable for him and he upgraded to
kernel 2.4.19 and is fine now. I have heard some similar input from a
few other people. While discussing this in another forum I was told that
the 2.4.19 kernel which is in the testing /v2.4/testing at
ftp.kernel.org is just a patch. Now my friend's system reports the
kernel as version 2.4.19 and as far as I can tell when I look at the
version report it looks like a whole new kernel and not a patch of
2.4.18.

I would assume a patch just installs new code in a few specific areas of
the kernel rather than updating the majority of the functions in the
kernel. This seems as though it could be considered a new kernel though.
My limitation of kernel structure I believe is my weak point here.

I guess my question is, what is the definition of a patch vs a new
kernel?

I know I could look this up and read on it but my time is rather limited
at the moment so I was hoping for a nutshell type answer that will give
me a overall picture. The fine points I will fill in later.

Craig S.