Almost all applications show an icon in the dock with a little light blue dot underneath.
Hitting Command +Tab will cycle through all open apps.
Some can run in the background and can be seen in the Activity Monitor. It's like the task manager in Windows.
EDIT: sorry, I just re read your post. There is a way to view what apps have been opened, but as for what times, maybe a system log would track it. I'm not a my Mac right now to check my logs.