Differences between the different installers of SQL Server 2014 Express
Express 32BIT WoW64\SQLEXPR32_x86_ENU.exe 


  This is for installing the 32-bit version of SQL Server Express, just the engine, on a 64-bit OS in WoW64 mode. WoW64 mode is described in excruciating detail here.

Express 32BIT\SQLEXPR_x86_ENU.exe 


  This is for installing the 32-bit version of SQL Server Express, just the engine, on a 32-bit OS.

Express 64BIT\SQLEXPR_x64_ENU.exe


  This is for installing the 64-bit version of SQL Server Express, just the engine, on a 64-bit OS.

ExpressAdv 32BIT\SQLEXPRADV_x86_ENU.exe


  This is for installing a 32-bit engine and advanced services on a 32-bit operating system. "Advanced services" are explained here.

ExpressAdv 64BIT\SQLEXPRADV_x64_ENU.exe


  This is for installing a 64-bit engine and advanced services on a 64-bit operating system. "Advanced services" are explained here.

ExpressAndTools 32BIT\SQLEXPRWT_x86_ENU.exe 


  This is for installing a 32-bit engine and tools (most importantly SSMS and Profiler) on a 32-bit operating system. "Express With Tools" is explained here.

ExpressAndTools 64BIT\SQLEXPRWT_x64_ENU.exe 


  This is for installing a 64-bit engine and tools (most importantly SSMS and Profiler) on a 64-bit operating system. "Express With Tools" is explained here.

LocalDB 32BIT\SqlLocalDB.msi 


  This is for installing LocalDB on a 32-bit operating system. LocalDB is described here.

LocalDB 64BIT\SqlLocalDB.msi 


  This is for installing LocalDB on a 64-bit operating system. LocalDB is described here.

MgmtStudio 32BIT\SQLManagementStudio_x86_ENU.exe


  This is for installing a standalone Management Studio on a 32-bit operating system.

MgmtStudio 64BIT\SQLManagementStudio_x64_ENU.exe


  This is for installing a standalone Management Studio on a 64-bit operating system.