When you open an R18 file in R19, you will be able to open and edit everything about that file as if you were working in R18. In between releases sometimes Dassault changes algorithms around, thus some features may behave slightly differently under the hood in how they create geometry. If you wish to take advantage of these (at the minor risk the shape of your part will be very slightly different possibly causing some features to fail down the tree) right-click in your Tree and choose "Upgrade".
When you open an R19 file in R18, you won't be able to open it nor edit it. If the user running R19 runs the Backwards Compatibility you will be able to open and edit that file in R18 but the Part feature tree will be gone. If you're talking assemblies, constraints and any in-context references will be gone. Drawings as far as I know will be transfered but will have no more link to their 3D part or assembly, re-establishing this link is possible but will most likely lead to dimensions dangling (so you will need to re-attach them) and possibly detail views, sections views and such may need to be recreated as well.
In short, upwards compatibility (R18->R19) is easy but downwards compatibility (R19-R18) is a major pain.
Certified SolidWorks Professional
