The other replies are pretty much taking care of your questions about MIDlet Suites but I would suggest reading the MIDP specification to get the information first-hand:
http://www.jcp.org/en/jsr/detail?id=118
On recent phones (Series60 5th edition from Nokia Sony-Ericsson and Samsung, other Sony-ericsson phones), the Content Handling API (CHAPI) is supported for MIDlet-to-MIdlet invocation.
That's JSR-211. You can read the specification for that here:
http://www.jcp.org/en/jsr/detail?id=211
The latest version of the Nokia Java Developers Library will contain more explanation:
http://www.forum.nokia.com/Tools%5FDocs%5Fand%5FCode/Documentation/Java/
The latest JavaME SDK will contain CHAPI code examples:
http://java.sun.com/javame/downloads/sdk30.jsp
Edit: CHAPI is also available on non-Symbian Nokia phones from Series 40 5th Edition, Feature Pack 1 onward, according to the Nokia Java Developers Library. You can find which nokia phone is running which version of Series40 at:
http://www.forum.nokia.com/devices/matrix%5Fall%5F1.html