Apple Lossless is one of many lossless audio compressors, meaning that it can achieve a certain level of file compression without compromising the audio data at all in the process. Unlike WAV and AIFF files, Apple Lossless files support full tagging with metadata, and output file sizes tend to range from roughly 50-70% of the size of uncompressed WAV or AIFF files.
As far as the consistency of the compression, it’s lossless, so unless some bizarre error occurs during compression (VERY unlikely), the output is always the same as what it would be if you were encoding directly to WAV or AIFF. 