18 is still a version number. it means that there are some change, but change is not bigger enough to make it a 2.6. just like the changessss in 2.6 is not big enough to make it a 3.0.
for the 14. it is a customized number, u can use any number or letter. here it is set by redhat. means redhat have 14 minor modifications on it.
2, 4, 18, is decided by linus. and -14 is decided by u.