It is minified version. It is exactly the same as normal file, but without extra spaces and comments, reducing its file size.
You can just use latest one. Currently its 1.6.1, which you can download from jquery.com. It is backwards compatible with old versions, so if your scripts are written for old versions of jQuery, they should work with 1.6.1 without problems.
Or you can just link to http://code.jquery.com/jquery-latest.min.js that always uses latest version.