from the Lightroom help file (emphasis mine, below)
About color management in Lightroom
Lightroom simplifies color management in your photographic workflow. You donít need to choose color settings or color profiles until you are ready to output your photos. To take advantage of Lightroom color management, you need to calibrate your computer monitor so that you are viewing accurate color. See Calibrate and profile your monitor.
Color spaces, color profiles, and tonal response curves
Itís not necessary for you to understand how Lightroom manages color internally, but the following information may be useful in your workflow.
A color space describes a range or gamut of colors. Various devices in your photographic workflow have different color gamuts in which they can record, store, edit, and output photos. A color profile defines a color space so that Lightroom knows how to manage and convert colors in your photo.
Raw photo files generally donít have embedded color profiles. For raw files, the Develop module assumes a wide color space based on the color values of the ProPhoto RGB color space. ProPhoto RGB encompasses most colors that cameras can record.
A color profile is also defined by a gamma value, or more accurately, its tonal response curve. The tonal response curve defines how tonal values in the raw image are mapped. To provide useful information in the histogram and RGB value display, Lightroom assumes a gamma value of approximately 2.2. More accurately, it uses a tonal response curve similar to the tonal response curve of the sRGB color space.
While Lightroom uses a tonal response curve to provide information for the histogram and RGB values, it manipulates the raw data before it is tone mapped. Working in this linear gamma avoids many of the artifacts that can result in working with a tone-mapped image.
The Library module stores previews in the Adobe RGB color space. These previews are also used when printing in draft mode.
For rendered files such as TIFF, JPEG, and PSD files, Lightroom uses the imageís embedded color profile to display the image, histogram, and color values. If the image doesnít have a profile, Lightroom assumes the sRGB profile, and the image may not look as expected on your monitor.
Output color profiles
When you print a photo in Lightroom, you can choose to convert the colors to more closely match the color space of the printer, paper, and ink you are using. For information on working with printer color profiles, see Set print color management.
Lightroom automatically exports images in the Slideshow and Web modules using the sRGB profile so that the color looks good on the majority of computer monitors.