iCloud makes it easy to share calendars with others. You can create a public calendar that is read-only to others and anyone can subscribe to it. You can also create shared calendars that can be viewed and/or updated by a list of others you invite.
Reminders are calendar events that can be used as to-do items or alerts. They sync between your Macs and iOS devices. They can also be set as location-based alerts on the iPhone. You can use Siri to create new reminders on the iPhone 4S.
See how you can use iCloud to view the same contacts, calendars and bookmarks across you Macs and iOS devices. You can also work on Pages, Numbers and Keynote documents across iOS devices and access them on your Mac as well.