Files and paths are explained in detail here: http://solidity.readthedocs.org/en/latest/layout-of-source-files.html
import "filename";, filename is always treated as a path with / as directory separator, . as the current and .. as the parent directory. Path names that do not start with . are treated as absolute paths.
To import a file x from the same directory as the current file, use import ”./x” as x;.
Use in actual Compilers
When the compiler is invoked, it is not only possible to specify how to discover the first element of a path, but it is possible to specify path prefix remappings so that e.g.
github.com/ethereum/dapp-bin/library is remapped to
/usr/local/dapp-bin/library and the compiler will read the files from there. If remapping keys are prefixes of each other, the longest is tried first. This allows for a “fallback-remapping” with e.g.
“” maps to