Not All Yum Repositories Will be Enabled by Default

You may think the repositories in /etc/yum.repos.d are all useful and Yum will use all of them as a base to operate packages by default. In fact, Yum will use only the repositories which are enabled in the configuration file.

If you would like to know which repositories are enabled, you can list them in this way:
[root@www ~]# yum repolist
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
 * base: mirrors.grandcloud.cn
 * epel: mirrors.yun-idc.com
 * extras: ftp.tc.edu.tw
 * updates: mirrors.hustunique.com
repo id         repo name                                                 status
CentALT         CentALT Packages for Enterprise Linux 6 - x86_64             323
base            CentOS-6 - Base                                            6,367
*epel           Extra Packages for Enterprise Linux 6 - x86_64            10,606
extras          CentOS-6 - Extras                                             14
updates         CentOS-6 - Updates                                           655
repolist: 17,965
In the above case, I noticed that two repos "remi" and "remi-test" are not listed, so I checked the configuration file.
[root@www ~]# vi /etc/yum.repos.d/remi.repo
[remi]
...
enabled=0
...
[remi-test]
...
enabled=0
...
And I replaced the value of "enabled" from 0 to 1 on both repos.
enabled=1
Let's list all available repositories again. [root@www ~]# yum repolist
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
 * base: mirrors.grandcloud.cn
 * epel: mirrors.yun-idc.com
 * extras: ftp.tc.edu.tw
 * remi: mirrors.hustunique.com
 * remi-test: mirrors.hustunique.com
 * updates: mirrors.hustunique.com
repo id      repo name                                                    status
CentALT      CentALT Packages for Enterprise Linux 6 - x86_64                323
base         CentOS-6 - Base                                               6,367
*epel        Extra Packages for Enterprise Linux 6 - x86_64               10,606
extras       CentOS-6 - Extras                                                14
remi         Les RPM de remi pour Enterprise Linux 6 - x86_64              1,465
remi-test    Les RPM de remi en test pour Enterprise Linux 6 - x86_64         24
updates      CentOS-6 - Updates                                              655
repolist: 19,454
Now, they are available and listed.

If you just want them to be enabled only at run-time, you can add an option "--enablerep" like this:
[root@www ~]# yum --enablerepo=remi,remi-test info mysql
...