what is meaning of layer ?
while making a sketch a new layer is created every time.every sketch have a different layer and that creates problem in doing further work.is there any solution to that?
As default most if not every feature is placed in the work layer. And the work layer stays the same unless you change it manually.
How does it appear that every sketch goes to a different layer? Can you upload a screenshot from the Part Navigator and the Layer Settings (Ctrl + L) window? Because I suspect it's something else than layers that's making problems for you.