yum update update packages and system software, system kernel
yum upgrade only update package does not update the system software and system kernel
View the version number
1 [root@localhost ~]# uname -r 2 3.10.0-957.27.2.el7.x86_64
Where el represents the Enterprise Linux, el7 representatives RHEL7
x86_64 architecture for the system: 64-bit systems
3: major version number
10: minor version number, represents a stable version (because there are an even number)
0: The number of revision, expressed modified, the first two numbers combined together can be described in the kernel series. The stable version 3.10.0, which is version 3.10 kernel series.
957: Indicates the current number of kernel patches