SQL Express core has three packages available while SQL Express w/ Tools and SQL Express Advanced have two each.
The SQL Express family:
SQLEXPR32_x86_ENU.exe / SQLEXPR_x86_ENU.exe
SQLEXPR identifies the core edition, the addition of the 32 means that it is a 32 bit only package. (We do this in order to reduce the size for deployment.)
Without the 32 means it supports installing in the WoW subsystem on an x64 platform, but it's still the 32bit version of SQL Express.
x86 is used to identify that this is the 32bit version of SQL Express.
ENU identifies the language, English here. Different languages will have different three letter suffixes.
x64 is used to identify that this is the 64bit version of SQL Express, so it can only be installed on x64 systems. We don't support installing SQL Express onto IA64 computers.
SQLEXPRWT_x86_ENU.exe / SQLEXPRWT_x64_ENU.exe
These are the package names for SQL Express with Tools.
There is no 32bit only package for this edition, just the 32bit WoW package.
SQLEXPRADV_x86_ENU.exe / SQLEXPRADV_x64_ENU.exe
Same as above but for SQL Express Advanced.
All these packages have prerequisites which are detailed in earlier posts in this blog and on the download pages for each edition. If you have any questions post them in the SQL Express forum on MSDN.
This Info Shared From MSDN Blog