Crontab past the hour

The first one will (with most common cron implementations) run the command every 15 minutes and is equivalent to 0,15,30,45 * * * *.

The second one will run 15 minutes past the hour, every hour.

This is described in the crontab(5) manual on your system (man 5 crontab).

Your first crontab line means running a command every 15 minutes.

The second one means running a command 15 minutes after every hour.