iTunes groups albums together solely by album name.
Lets say you're importing a 2-CD set. iTunes will likely give each CD a different album name, appending "Disc 1" or something like that to the end. It will also start numbering each CD's tracks with "1". You would like to have both CD's in the same album set, with all the tracks in the proper order.
You need to edit the track numbering for the second disc -- hit "Get Info" on the first disc, change the track number and total tracks number to the proper value, then hit "next" to go to the next track. After that, If you select all the songs in an album and hit "Get Info", you can change the value in the "album" field for all the selected songs. If you change it to the same name as Disc 1 (or change both discs to the same name), then iTunes will group them together.
I do this all the time because I select albums by artist, and if there's a track on a CD with a guest artist, iTunes will list the artist as "Foobar Band & Guest". When I select "Foobar Band", this track doesn't show up, so I change the name if the track to make everything match.