ANSI And Unicode
Since version 5.1 in October 2008, Notepad++ came in two distinct builds: ANSI and Unicode. This acknowledges the fact that Windows itself has gone Unicode as well.
The change added support for Unicode characters in paths, which many users in countries english is not the native language of are faced with. Additionallly, the change made it possible for Notepad++ to be localised in any language, including those with DBCS script.
For more information on Unicode, see http://www.unicode.org.
Unicode is now the only development path for Notepad++. ANSI was dropped starting with version 6.0 in april 1012. There was only sporadic evidence of using ANSI builds anyway, and nearly all existing ANSI plugins had been converted to Unicode or superseded. More detail on ANSI-only plugins in the dedicated section of Plugin Central.
While you can still build an ANSI binary from sources, it will not contain features introduced with v6.0. It may not work properly at all when some critical code is provided without a preprocessor guard for Unicode builds.
This page itself is bound to be deleted at some ponint in the future, when it is felt that it describes a temporary, foregone state of coexisting builds, which had lasted for 3 years and a half.