I won’t try to explain the difference between a codec and a container. You just need to know that aac is a codec and mp4/m4a is a container. And also there is no technical difference between MP4 and M4A. M4A is created by Apple just to differentiate between video and audio files inside MP4 container. (Wiki articles: codec, container)
Why it is a good idea wrap raw aac files into M4A container? Because some music players only supports certain file types and aac is not a popular one, one other problem is that aac does not have a standardized documentation for tagging songs. So if you tag your files in one application another application may not be able to read these.

Lately I have been using a wonderful stream saver application called StreamWriter with which you can listen online radios and at the same time save the songs, you can even specify some keywords and StreamWriter will save every song that matches your keywords depending on their tags whether or not you are listening them at the time. Most of my favorite radios are streaming with aac format since it is more efficient than mp3. The problem with aac is like I explained above its tagging features (or lack of them). I cannot use auto tagging programs like MusicBrainz Picard with this format. So I decided to convert all my aac files to M4A without losing any quality. And since aac is a raw format and M4A is only a container it is possible.

Wrapping process:
Download MP4Box for wrapping the files and Batchenc for automating the process for all the files.
Extract all the files in one folder.
Open Batchenc and drag all the files onto it.

Write:

MP4Box.exe -add <infile> <outfile.m4a>
in Command line area, you can change m4a to mp4 if you like (you can also add this line in Batchenc_presets.cfg for future uses – open cfg file in notepad).
If you want you can specify a different output directory.
Click start and few seconds later you are done.

I have over 4000 mp3s on my hard disk. And I don't like burning mp3s to DVD with missing tags.
Most of my mp3s come from offline satellite download and they came named similar as below:
193.251.135.116_08C9A07485__66477.mp3
You have to rename them one by one according to their tags. This is the easy part since most of the media players can consolidate your library automatically.

* You can consolidate your music library with Windows Media Player by going to options and then enabling the rename and rearrange music using media information.

Main problem is with files without tags or with incorrect tags. This is where MusicBrainz Picard comes to rescue. By matching acoustic fingerprint of your files with its database, it correctly identifies and tags your mp3s. You can also set it to rename your files too in the process.
How to use it:
First adjust the program to your needs.
I enabled embedding cover images to tags and renaming file names.
Under 'View' enable 'File Browser'.
Select a folder which your files reside. Drag the folder under to the Unmatched Files in the center. You can drag the entire folder or choose files individually.

The tagger will read the metadata from each of the files and unless they have been tagged before, the files will be deposited into the "Unmatched files" folder. Files that have been tagged before and contain the MusicBrainz track identifier will be opened up as releases in the right-hand pane.
Once the tagger finishes processing the files, press the "Cluster" button (white square with 'clusters' on them). This will cause the tagger to attempt to group the files into album clusters by examining the metadata read from the files and clustering files that appear to belong to the same album. Files that are not matched into album clusters will remain in the "Unmatched files" folder. In future the tagger will automatically cluster the files when it has finished reading the metadata from the files.
If your existing files do not have an album tag, click on unmatched files to select all remaining files then click scan. All the matching tags will be shown on right pane. You only have to select the matching files and click save.
How to tags files with picard
Advanced tutorial
Musicbrainz server currently has info of 7138615 tracks.
You can also use album art plugin to add album arts in tags.
As an alternative you can also use Winamp for identifying your music files, it has a auto-tag feature which uses Gracenote database to find track information. You can access it by right clicking on any track / send to / auto tag.