Where can I find a summary description of what plugins do? What are their features? Its a pity that the Plugin Manager does not give such info. I searched the Sonar documentation, but cannot find this.

You are seeking to learn the larger categories of plugins and what they do?
 
Like...
What does a compressor do?
What does an expander do?
What does reverb do?
What does delay do?
What does multi-band mean?

Is this what you're looking for?
SONAR AUDIO Effects

Aaah - I should have been too clearer.
Plugins have names. Some names are obvious as to what their functions are. If 'reverb' is in the name, it is very obvious what its function is. But it would be nice if its main features are shown in the Plugin Manager - so that we could at a glance see how all the different types of reverb plugins do.
And then there are those plugins with names that give no clue at all as to what they do: Z3TA, TH3, PSYN, sfz, RTP etc.
By chance I stumbled on a short description on the web about Z3TA - but it is very inefficient to search for the meanings of plugins.
LibreOffice gives descriptions of their plugins -
Something similar for Sonar would be a big help.

Did you click on the link in blue "SONAR AUDIO EFFECTS"? 

you can always load them to have a quick butchers, the gui usually makes it clear

Jacques,
 
Sounds like you're just starting out. The fact is, you need to do your research into each and every plug in. Those included with SONAR and every third party plug you purchase. We all have. It takes a serious investment of time, but is necessary and worth it in the long run.
 
What I've done in Plugin Manager, is set up a custom list, grouping plugins by type. For instance: all reverb type plugs go in a group titled Reverb, etc. Makes it much easier to locate a specific plug.
 
Best of luck and always come back here. Lots of very helpful members.

It's easy to forget since software is so intangible but if you bought a studio with 90 rack based fx and sound modules you'd also need to learn what each one does.

There are a couple of problems with implementing what you apparently want. 
 
The first is that SONAR itself is pretty much agnostic about what a plugin is doing. All it is likely to see is a plugin identifier and a basic way to link and coordinate, a location to deposit track output and another location to retrieve processed data, What actually is happening to the data in the black box that is the plugin is a mystery to SONAR as it is to you. Since Cakewalk does not make every available plug, and since there is no standard way for the plugin formats to contain a verbose description of what it does, SONAR would have no way of providing you with such descriptions. A metadata arrangement in the plugin manager, which could link the plugin identifier to a comment about its use might be useful, but how would that information be populated if not by your own comments?
 
The second is that some plugins defy stereotypical descriptions. Some are doing so many things (or such odd stuff) that reading the multi-paragraph descriptions provided by their developers, or more often the misleading descriptions provided by their sales departments. and even reading the manuals or documentation do not give you a clear understanding of how they are going to mangle/improve your sound. Sometimes you just have to get into them and muck about for a while to get that information. Once having done that, a comment written by you might be useful, but that might as well be in a notebook or database that you maintain as by one that is  linked to SONAR.