Generally whats the difference between the 2?
On what situation should i use polyline? can i convert line into polyline?
What can line do that polyline can't?

Line is just one straight segment, Polyline is a series of segments (lines or arcs). Line can be 3D, plain Polyline cannot (there are also 3DPolylines).
 
You can convert Lines to Polylines (e.g. with PEDIT) and Polylines to Lines (with EXPLODE).

thanks

A rectangle created with the line command consists of four separate entities.  The sample rectangle created with one continuous polyline would be treated by AutoCAD as a single entity.  Polylines, via the Pedit command, can also be turned into splines and can have curves fitted to them.

It's often more what a closed polyline object can do that a series of individual line segements cannot:  Provide direct computation of perimeters and areas.  Those can be worth the price of admission alone.

The distinction between line and polyline is.
line = 2D = ok.polyline = 3D = ok.
and can convert polyline by command explode.
Therefore if you use just drafting 2D will not have to use the polyline.

Avoiding the use of polylines in 2D drafting is not a good CAD drafting practice.  

Hi, The original Polyline in AutoCAD while a vast improvement over having to use line segments and arcs was a resource hog. each vertex and end were separate entities so a Polyline with two vertexes was four entities. In r14 AutoCAD introduced the lwpolyline (lightweight polyline) which was a single entity. For drawing which included lots of contours or other complex polylines drawing sizes shrank considerably. Big difference in speed and stability as well. Thanks!________________pest exterminator