You should find information here:
- MobileMe: About moving to iCloud
- iCloud: Features that work with one account at a time on iOS 5 and OS X
- iCloud: How to back up iCloud data
- iCloud: Backup and restore overview
- iCloud: About storage buffering
- Using and troubleshooting Back to My Mac with your iCloud account on OS X Lion
- iCloud: Troubleshooting Find My Friends
- iCloud: Calendar Events, Reminders, To Dos, and Tasks behavior differs by application
- iCloud: Distinguishing between Calendars and Reminders
- iCloud: Troubleshooting iCloud Reminders and Tasks
- iCloud: Manually importing data from iCal to iCloud Calendar
- iCloud: Understanding where iCloud Calendars are displayed in iCal
- iCloud: Reminders are called Tasks in Microsoft Outlook on Windows
- iCloud: About the iCloud Web App Plugin for Internet Explorer
- iCloud: Troubleshooting web browser issues with icloud.com
- iCloud: Troubleshooting iCloud Bookmarks
- iCloud: How spam is filtered
- iCloud: Why was I spammed?
- iCloud: Keeping the Junk folder consistent between webmail and OS X Mail
- iCloud: How to get long email headers for iCloud accounts
- iCloud: About email aliases
By most popular definitions, a network cloud is just network accessed storage and/or software, so I'd say sure, it is a "true" cloud service. A network cloud can be utilized for a wide array of things, from simple storage, backup or file sync, to hosting remote applications, shared resources or whatever.
iCloud is a network accessed storage service for sync'ing Apple device and software content.
A "cloud" in computing terms is simply an internet-based server on which your data is stored and where applications that provide that data run.
MobileMe and iCloud are the same in that respect. They both store data on a remote server from where you can access/sync it.
As far as limitations, you need to be a bit more specific in your question. iCloud does a lot, and each of its various functions have their own individual limits.