You could start SAP Web IDE from the following URL. Register is needed.
https://account.hanatrial.ondemand.com/cockpit
You could also get more information in the following URL.
http://scn.sap.com/docs/DOC-55465
It is based on Eclipse Orion.
What is Orion? It is a web IDE, a IDE in you browser.
It is not a plugin of traditional Eclipse IDE.
You could treat Orion and traditional Eclipse as two different IDEs.
Although they have something in common.
You could get some ideas about Orion by the following URLs.
https://wiki.eclipse.org/Orion
https://wiki.eclipse.org/Orion/Documentation/User_Guide/Getting_started
https://wiki.eclipse.org/Orion/FAQ
https://www.eclipse.org/org/press-release/20141027_cloud_initiative.php
http://eclipsesource.com/blogs/2011/12/23/introducing-eclipse-orion/
So the idea of SAP Web IDE is similar. But you could use it to write codes for sapui5, hana... on the Hana Cloud Platform.
Of course, you could install a local copy of SAP Web IDE. If you are interesting in doing it, please follow the guide in the following URL. After the installation, you could access you local SAP Web IDE server by browser. (Yes, because it contains a web server.)
http://scn.sap.com/docs/DOC-58926
The document for SAP Web IDE.
https://help.hana.ondemand.com/SAP_RDE/frameset.htm
Some other interesting URLs.
About SAPUI5 ==> https://sapui5.hana.ondemand.com/sdk/#docs/guide/058c7fd461d94223a11ef4c89b2259ea.html
Home page about Hana Cloud Platform. ==> http://hcp.sap.com/index.html
Another web IDE based on Orion. ==> https://orionhub.org/
SAP Development Tools for Eclipse. This is a plugin for normal eclipse. ==> https://tools.hana.ondemand.com/