All edits are stored within the Lightroom Catalog. They are completely reversable at anytime. There is no need to “save” your work unless you want access to the edits outside of Lightroom.
You can write external XMP edits for individual files while in the Develop Module. Go to Photo>Save Metadata to File, ⌘-S (Mac) or Ctrl-S (PC). This writes a sidecar XMP file to the hard drive for raw images. For JPEG, TIFF, PNG and PSD files, metadata by defualt is embedded in the file.
XMP data can be automatically saved for all images with a preference change in the Catalog Settings.
Go to Lightroom>Catalog Settings, Metadata tab. Choose “Automatically write changes into XMP.” Think twice before you do this though, because it could slow down your workflow as Lightroom continually saves data to your hard drive as you edit.
XMP edits saved externally can be viewed by Adobe Bridge.