Once, PostScript was the exchange format for vector data. Now, however, it has largely ceded its position to PDF, which supports all practically useful features of PostScript but is at the same time richer (most importantly, it supports transparency) and much easier to support in software. So, whenever you have the choice, use PDF instead of PostScript; however, a lot of old projects and clipart exist as PostScript files, and there’s still a lot of obsolete software around which either does not support PDF at all or supports it poorly while trying its best to understand PostScript.
Note
EPS is PostScript with some additional limitations which make it better adapted to being imported and inserted into other documents. An EPS file is always a single page, always has all fonts and bitmap images embedded (regular PS files are not obliged to embed stuff, although it is preferable that they do it) and its page size is always exactly clipped to its contents.
Inkscape supports PS and EPS export natively. The dialog has much the same options as the PDF export dialog:
You can choose the level (i.e., version) of PostScript; most modern software and printers support Level 3. Fonts are always embedded into PS or EPS export (unless you choose to convert them to paths), and filters can be optionally rasterized, just as they are in PDF export. Objects with nonzero opacity are also rasterized automatically (you cannot suppress it) because, unlike PDF, PostScript does not support vector transparency.
For PS and EPS import, you have to install an additional piece of software, the Ghostscript interpreter, which is run automatically to convert the PS or EPS file into PDF format, which is then fed to Inkscape. At http://pages.cs.wisc.edu/~ghost, you will find its versions for all major operating systems. Make sure the gs or (on Windows) gs.exe executable file of Ghostscript is in your PATH; if it is not the case, Inkscape will not even list PS and EPS in its file format list in the Open or Import dialogs.
Since PS/EPS import ends up as PDF import from the Inkscape viewpoint, you will see the same PDF import dialog (Figure B-2). In particular, if a PS file contains more than one page, this dialog will allow you to choose which page to import.