Apache is OK, but what is this in error.log - [mpm_prefork:notice]?

The log file just shows some startups/shutdowns of Apache workers. In your Apache configuration, you can set how many workers (aka threads) Apache may use. On a regular setup, Apache can be started several times. Especially when your server is busy (e.g. there are many visitors on one of your vhosts), it's not strange to see 20 (or more) Apache processes running. There is nothing to worry about, they're just informational.

[mpm_prefork:notice] [pid 8817] AH00163: Apache/2.4.7 (Ubuntu) PHP/5.5.9-1ubuntu4 configured -- resuming normal operations

This means a new thread spawned under process id 8817.

[core:notice] [pid 8817] AH00094: Command line: '/usr/sbin/apache2'

This is just a notice saying that the /usr/sbin/apache2 command was used to start the thread, so no special flags/options were passed to it.

[mpm_prefork:notice] [pid 8817] AH00169: caught SIGTERM, shutting down

This line tells you that the process with process id 8817 has shut down again.

[Updated] This log was invoked by /etc/apache2/apache2.conf which it has the directive pointing to the log file that you are seeing at. To see less of the logging, you can edit this line in apache2.conf LogLevel warn to LogLevel error which will log only if there is error that causing the server to malfunction. Options for that logging are: trace1, debug, info, notice, warn, error, crit, alert, emerg. You can play with all of these options to find which logging level you like the most.

To see the log for your domain, you can find that in your conf file that hold the configurations for the domain host in /etc/apache2/sites-available. I.e. yoursite.conf