A schedule agreement contains details of a delivery schedule but a contract just contains quantity and price information and no details of specific delivery dates 

What's the difference between schedulling agreement with normal order? 
What's the condition for us to choose schedule line or order? 

Both of them contains schedule line, price, quantity. 

There are a couple major differences: 

(1) - Schedule agreements allow you to have 2 different sets of schedule lines (VBEP-ABART).  Standard SAP you should have two sets of tabs - of schedule lines. One Forecast & the other JIT.  Forecast forwards the schedule lines to planning (seen in MD04) and JIT passes them to shipping (VL10). They can be identical or different. Typically these are used for component supplier customers (namely Automotive). The customer will provide you 4-10 weekly buckets (usually a 
Monday date) of future forecast qtys. Also send you 1-2 weeks of individual FIRM ship dates - which are entered on the JIT. It comes down to the customer not knowing exactly what they need next week, but they don't want to suprise you with a large order qty, where your lead times are 5+ days. The forecasted qtys they sent last week should account for this. 

(2) Cumulative Quantities are tracked and influence how the schedule agreement passes requirements to both forecasting and shipping. These qtys are sometimes requested by the customer on ASNs. Cumulative qtys reset at year end unless you've got a customer calendar or you've modified standard SAP userexits to not reset. 

Schedule agreements are very nice when the customer sends EDI data (830s = forecast or 862s = JITs). Outside of that they can really cause trouble regarding daily maintenance, missing requirements, cum qty corrections, year end processing, etc. 
One alternative would be to use customer independent requirements - entering the weekly, monthly forecasting qtys and entering standard sales orders (with or without multiple schedule lines) to represent the true firm qtys.

Do you have a SAP SD Question?

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.sap-img.com