XBuild is the Mono specific version of the MSBuild utility.
XBuild is Mono's implementation of msbuild and it allows projects that have an msbuild file to be compiled natively on Linux and OS X. xbuild has been part of the standard Mono distribution for some time now, but it is not 100% complete yet.
XBuild supports C# and VB.NET projects out of the box. It also supports ASP.NET Projects, both WebApplication and WebSite projects. It also supports custom tasks, targets, extension of the build system through Before/After* hooks (eg. Before/AfterBuild) and *DependsOn properties (eg. BuildDependsOn).
Microsoft open sourced MSBuild at GitHub and ported it to non-Windows platforms. Starting from Mono 5.0 release, xbuild has been obsolete and replaced by MSBuild.
Interestingly, XBuild was the code name of MSBuild when Microsoft worked on Visual Studio 2005 and .NET Framework 2.0.