There are two types of subreports
 a.Unlinked Subreport
       ie the subreport doesn't depend on the main report 
at all.
 
 b.Linked subreport

  A linked subreport is handy when we need to display 
related data elements from more than one database table on  
a report.

There are three types of subreports
a.Unlinked Subreport
       ie the subreport doesn't depend on the main report 
at all.
 
b.Linked subreport

  A linked subreport is handy when we need to display 
related data elements from more than one database table on  
a report
 
c.On-Demand Subreport

If your report has a section that handles a large number of 
records, you can put that section into an on-demand 
subreport. An on-demand subreport appears as a hyperlink in 
the primary report. When you open the primary report, no 
data is retrieved for the on-demand subreport until you 
drill down on the hyperlink.

agreed with answer of Swapna.
Just a bit more information. 
You can not schedule an On-Demand Subreport on Crystal 
Enterprise, as you can give server info only to main report.

if it as linked or unlinked report it will run along with 
the main report, where as if report is schedulled, you get 
a copy of it, at that time you will have to click on hyper 
link to get the report run. that time it will ask for 
server info which you cant give.
give it a try if you are not getting it fully......