The ASEC file extension is associated with the Google Android operating system. Android from version 2.2 Froyo supports installation of applications to memory card. Applications moved to memory card are encrypted and use ASEC file extension. ASEC files can be found in .android_secure folder saved on memory card.
Applications stored in ASEC encrypted file format cannot be modified by other applications installed in phone, or computer. Applications installed on SD card are paired with Android phone, where they have been installed. It is not possible to put SD card to other Android phone and run applications installed on SD card.
Only Android SDK Emulator that is a part of Android SDK Tools is able to run applications installed on SD card in ASEC encrypted format, if the Android phone with SD card is connected to the computer.
If you move application back from SD card to phone's local storage, Android also decrypted ASEC package file to standard APK application package.