Edits in LR are made using XML line commands that alter the algorithms LR uses to render the image.
In other words none of the pixels in the original are changed and LR edits are called "non-destructive".
Here is a LR image file with it's XML line commands:
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.3-c007 1.136881, 2010/06/10-18:11:35 ">
<rdf:RDF xmlns:rdf="
http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf: Description rdf:about=""
xmlns:tiff="
http://ns.adobe.com/tiff/1.0/"
xmlns:exif="
http://ns.adobe.com/exif/1.0/"
xmlns:dc="
DCMI Metadata Terms"
xmlns:xmp="
File management, metadata integration | Adobe Extensible Metadata Platform (XMP)"
xmlns:aux="
http://ns.adobe.com/exif/1.0/aux/"
xmlns: photoshop="
Photo editor | Download free Adobe Photoshop CC trial"
xmlns:xmpMM="
File management, metadata integration | Adobe Extensible Metadata Platform (XMP)"
xmlns:stEvt="
File management, metadata integration | Adobe Extensible Metadata Platform (XMP)"
xmlns:crs="
http://ns.adobe.com/camera-raw-settings/1.0/"
tiff:Make="NIKON CORPORATION"
tiff:Model="NIKON D300"
tiff:Orientation="1"
exif:ExifVersion="0221"
exif:ExposureTime="25/1"
exif:ShutterSpeedValue="-4643856/1000000"
exif:FNumber="16/1"
exif:ApertureValue="8/1"
exif:ExposureProgram="1"
exif:ExposureBiasValue="0/6"
exif:MaxApertureValue="30/10"
exif:SubjectDistance="4294967295/1"
exif: MeteringMode="3"
exif: LightSource="4"
exif:FocalLength="240/10"
exif:SensingMethod="2"
exif:FileSource="3"
exif:SceneType="1"
exif:FocalLengthIn35mmFilm="36"
exif:CustomRendered="0"
exif:ExposureMode="1"
exif:WhiteBalance="1"
exif:SceneCaptureType="0"
exif:GainControl="0"
exif:Contrast="0"
exif:Saturation="0"
exif:Sharpness="0"
exif:SubjectDistanceRange="0"
exif: DigitalZoomRatio="1/1"
exif: DateTimeOriginal="2011-06-04T12:16:39.94"
dc:format="image/x-nikon-nef"
xmp:CreatorTool="Ver.1.10"
xmp:ModifyDate="2011-06-04T12:16:39.94"
xmp:CreateDate="2011-06-04T12:16:39.94"
xmp:Rating="0"
xmp:MetadataDate="2013-09-20T14:17:15-05:00"
aux:SerialNumber="3015281"
aux:LensInfo="240/10 850/10 28/10 40/10"
aux:Lens="24.0-85.0 mm f/2.8-4.0"
aux:LensID="103"
aux:ImageNumber="4755"
aux:ApproximateFocusDistance="4294967295/1"
photoshop: DateCreated="2011-06-04T12:16:39.94"
photoshop:SidecarForExtension="NEF"
xmpMM: DocumentID="39E1B780355A7ACB536181A6802D6292"
xmpMM:OriginalDocumentID="39E1B780355A7ACB536181A6802D6292"
xmpMM:InstanceID="xmp.iid:1F32AC422922E3118535FE3A179F146B"
crs:RawFileName="6-4-11BlkSmthShpD300_0001.NEF"
crs:Version="6.7"
crs: processVersion="5.7"
crs:WhiteBalance="As Shot"
crs:Temperature="5900"
crs:Tint="-3"
crs:Exposure="0.00"
crs:Shadows="0"
crs:Brightness="0"
crs:Contrast="0"
crs:Saturation="0"
crs:Sharpness="25"
crs:LuminanceSmoothing="0"
crs:ColorNoiseReduction="25"
crs:VignetteAmount="0"
crs:ShadowTint="0"
crs:RedHue="0"
crs:RedSaturation="0"
crs:GreenHue="0"
crs:GreenSaturation="0"
crs:BlueHue="0"
crs:BlueSaturation="0"
crs:FillLight="0"
crs:Vibrance="0"
crs:HighlightRecovery="0"
crs:Clarity="0"
crs: Defringe="0"
crs:HueAdjustmentRed="0"
crs:HueAdjustmentOrange="0"
crs:HueAdjustmentYellow="0"
crs:HueAdjustmentGreen="0"
crs:HueAdjustmentAqua="0"
crs:HueAdjustmentBlue="0"
crs:HueAdjustmentPurple="0"
crs:HueAdjustmentMagenta="0"
crs:SaturationAdjustmentRed="0"
crs:SaturationAdjustmentOrange="0"
crs:SaturationAdjustmentYellow="0"
crs:SaturationAdjustmentGreen="0"
crs:SaturationAdjustmentAqua="0"
crs:SaturationAdjustmentBlue="0"
crs:SaturationAdjustmentPurple="0"
crs:SaturationAdjustmentMagenta="0"
crs:LuminanceAdjustmentRed="0"
crs:LuminanceAdjustmentOrange="0"
crs:LuminanceAdjustmentYellow="0"
crs:LuminanceAdjustmentGreen="0"
crs:LuminanceAdjustmentAqua="0"
crs:LuminanceAdjustmentBlue="0"
crs:LuminanceAdjustmentPurple="0"
crs:LuminanceAdjustmentMagenta="0"
crs:SplitToningShadowHue="0"
crs:SplitToningShadowSaturation="0"
crs:SplitToningHighlightHue="0"
crs:SplitToningHighlightSaturation="0"
crs:SplitToningBalance="0"
crs: parametricShadows="0"
crs: parametricDarks="0"
crs: parametricLights="0"
crs: parametricHighlights="0"
crs: parametricShadowSplit="25"
crs: parametricMidtoneSplit="50"
crs: parametricHighlightSplit="75"
crs:SharpenRadius="+1.0"
crs:SharpenDetail="25"
crs:SharpenEdgeMasking="0"
crs: postCropVignetteAmount="0"
crs:GrainAmount="0"
crs:ColorNoiseReductionDetail="50"
crs:LensProfileEnable="0"
crs:LensManualDistortionAmount="0"
crs: perspectiveVertical="0"
crs: perspectiveHorizontal="0"
crs: perspectiveRotate="0.0"
crs: perspectiveScale="100"
crs:AutoLateralCA="0"
crs:Exposure2012="0.00"
crs:Contrast2012="0"
crs:Highlights2012="0"
crs:Shadows2012="0"
crs:Whites2012="0"
crs:Blacks2012="0"
crs:Clarity2012="0"
crs:ConvertToGrayscale="False"
crs:ToneCurveName="Medium Contrast"
crs:ToneCurveName2012="Linear"
crs:CameraProfile="Adobe Standard"
crs:CameraProfileDigest="117885652CC92F92B2CEA3D4D5111301"
crs:LensProfileSetup="LensDefaults"
crs:HasSettings="True"
crs:HasCrop="False"
crs:AlreadyApplied="False">
<exif:ISOSpeedRatings>
<rdf:Seq>
<rdf:li>200</rdf:li>
</rdf:Seq>
</exif:ISOSpeedRatings>
<exif:Flash
exif:Fired="False"
exif:Return="0"
exif:Mode="0"
exif:Function="False"
exif:RedEyeMode="False"/>
<exif:UserComment>
<rdf:Alt>
<rdf:li xml:lang="x-default">(C) 2011 Keith Harrod Photography</rdf:li>
</rdf:Alt>
</exif:UserComment>
<dc:creator>
<rdf:Seq>
<rdf:li>Keith Harrod</rdf:li>
</rdf:Seq>
</dc:creator>
<dc:rights>
<rdf:Alt>
<rdf:li xml:lang="x-default">2011 Keith Harrod</rdf:li>
</rdf:Alt>
</dc:rights>
<xmpMM:History>
<rdf:Seq>
<rdf:li
stEvt:action="saved"
stEvt:instanceID="xmp.iid:17DDD069A25AE21197FFF56B1DD92706"
stEvt:when="2013-01-09T15:20:38-06:00"
stEvt:softwareAgent="Adobe Photoshop Camera Raw 6.7 (Windows)"
stEvt:changed="/metadata"/>
<rdf:li
stEvt:action="saved"
stEvt:instanceID="xmp.iid:1F32AC422922E3118535FE3A179F146B"
stEvt:when="2013-09-20T14:17:15-05:00"
stEvt:softwareAgent="Adobe Photoshop Camera Raw 6.7 (Windows)"
stEvt:changed="/metadata"/>
</rdf:Seq>
</xmpMM:History>
<crs:ToneCurve>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>32, 22</rdf:li>
<rdf:li>64, 56</rdf:li>
<rdf:li>128, 128</rdf:li>
<rdf:li>192, 196</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurve>
<crs:ToneCurveRed>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurveRed>
<crs:ToneCurveGreen>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurveGreen>
<crs:ToneCurveBlue>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurveBlue>
<crs:ToneCurvePV2012>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurvePV2012>
<crs:ToneCurvePV2012Red>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurvePV2012Red>
<crs:ToneCurvePV2012Green>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurvePV2012Green>
<crs:ToneCurvePV2012Blue>
<rdf:Seq>
<rdf:li>0, 0</rdf:li>
<rdf:li>255, 255</rdf:li>
</rdf:Seq>
</crs:ToneCurvePV2012Blue>
</rdf: Description>
</rdf:RDF>
</x:xmpmeta>