Serie C is the name of the third and fourth highest football leagues in Italy. It is broken up into Serie C1 (third tier) and Serie C2 (fourth tier).
In turn, Serie C1 consists of two divisions: Serie C1/A and Serie C1/B. For the 2004 season, C1/A has 19 teams, while C1/B has 18. At the end of the season, four teams (two from each division) go up to Serie B and six teams (three from each division) go down to Serie C2.
Serie C2 consists of three divisions: Serie C2/A, Serie C2/B, and Serie C2/C. For the 2004 season, C2/A has 18 teams, C2/B has 20, and C2/C has 18. At the end of the season, six teams (two from each division) go up to Serie C1 and nine teams (three from each division) go down to Serie D.
At the top is the Lega Nazionale Professionisti ('National Professional League', often referred to as 'Lega Calcio'), which has two divsions (Serie A and Serie B).
Below that is the Lega Professionisti ('Professional League'), which has five divisions: two in Serie C1 (the upper level) and three in SerieC2 (the lower level).
Below that is the simply named Serie D, a league of nine regional divisions that is organised by the Comitato Interregionale ('Interregional Committee').
Catania was relagated to serie C in May, promoted to serie B in June, relagated to serie C again in July and promoted back to serie B in August.
Calcio is in chaos this summer with the unprecedented decision to extend Serie B from 20 to 24 teams.
I presume the main reason why serie B teams have a problem with a 24 team league is that it would make it more difficult to win promotion to serie A (more competition) and more likely to get relegated (you would have to relagate at least 6 teams).