Mussolini's personal motivations in joining Germany in WW2 were entierly self serving, and not due to some ideological loyalty. Mussolini's goal was to expand Italy's power and influence through Europe and through colonies in Africa. Italy even had its own concept of Lebensraum, it was known as "spazio vitale" which means (like Lebensraum) "living space". Although the concept differed in that the lands Mussolini desired were considered, to some extent "Italian", where as the majority of the lands Hitler had no German and were to be made German through colonization. Mussolini wanted control over Nice, Corsica, Malta, The Dalmatian coast, Slovenia, Albania, and the Italian parts of Switzerland. He also sought control over Egypt and Sudan, as well as Tunis and the French and British parts of Somalia. Mussolini wanted unbridled control over the Mediterranean sea, and his foreign policy was geared towards achieving this. As for the territories that Mussolini didn't directly control? They would be used as buffer/puppet states; this was part of the reason Mussolini intervened in the Spanish civil war.

Mussolini and Hitler actually clashed in the early 1930's, almost exclusively over the issue of Austria. Hitler had been sizing up Austria for annexation since he got into power, and that clashed with Mussolini who saw Austria as a useful buffer state. Hitler eventually flew to meet Mussolini at the Venice Conference in 1934. I've written about that here At this point Hitler admired Mussolini as the founder of fascism, but the feeling was not mutual and Mussolini despised Hitler as an unoriginal, buffoon. The conference was a disaster and Mussolini's hatred of Hitler grew even more. Their relations soured even more when Austrian Nazis murdered the Austrian chancellor, Dolfuss, who was a close friend of Mussolini's. When Hitler again moved for Anschluss with Austria, Mussolini threatened war and moved his troops into the area, which caused Hitler to back off. Because of this Mussolini was viewed as a natural counter balance to Hitler's Germany, and Mussolini played both sides to try and get a favourable arrangement. Mussolini in 1935, along with Britain and France, signed an agreement, in a small Italian town called Stresa, that guaranteed the independence of Austria and formed a common front against German aggression. This would form the "Stress Front" which would be officially eliminated in 1939 (although it became meaniningless long before that).

Now this all changed when Mussolini visited Berlin in September of 1937, there he witnessed a parade set up by the German army, and came away convinced that Hitler was ready for war. Mussolini also became more and more ideological in his diplomacy. Where as before he had no issues dealing with the democracies of Britain and France, he began to drift close to Hitler. Mussolini's aggressive actions in Ethiopia, Spain, and Albania and his claiming of French lands also turned Britain and France against him. Mussolini eventually signed the Pact of Steel with Hitler in May of 1939, putting Italy in the Axis camp. The pact entailed the following:

Article I

Article 1 stated that Germany and Italy were to stay in communication with one another, in order to 'come to an understanding of all common interests or the European situation as a whole'.[2]

Article II

Article II obliged Italy and Germany to follow a similar foreign policy: for example, the two countries agreed, in the event of any 'international happenings',[2] to enter into mutual consultation.

Article III

Article III promised the full military support of the signatories should the other country go to war.

Article IV

Article IV supported the intentions of Article III, encouraging the establishment of greater cooperation in 'the military sphere and the sphere of war economy'.[2] This article also supported greater communication between Italy and Germany in order to achieve economic and military cooperation.

Article V

Article V compelled Italy and Germany to agree to all future armistices, further supporting increasing military planning between the two countries.

Article VI

Article VI of the Pact instilled the importance of maintaining relations with countries which were friendly towards either Italy or Germany.

Article VII

Article VII of the Pact of Friendship and Alliance between Germany and Italy dealt with the validity of the Pact: it stated that the Pact came into force upon completion and that it would last until 1949.

With this Italy was squarely in the Axis camp, but when push came to shove they did not join the war immediately in 1939. Mainly because Mussolini had been told that his army was not ready for war. Mussolini ignored Hitler's calls for cooperation, and instead began to formulate a plan of action. Mussolini payed lip service to Hitler, saying that he was "on his side, but that he couldn't fight just yet". In Secret Mussolini was planning to wait on the sidelines and let the war drag on for a few years and then jump on the winning side. It was only when France fell so quickly that Mussolini decided to join the war on the Axis side. Mussolini felt that the war would be over soon, since only Britain remained, and he didn't want to be left out of the peace talks. Mussolini felt that by committing himself to the Axis cause he would be given the British and French territories he had wanted.

Italy switched sides during WWII, if memory serves...was it able to significantly contribute to the Allied war effort after switching sides, or was it embroiled in internal conflict?

The allies did permit the new royal government of Italy to form a co-belligerent army. But two main factors hampered its development into an effective fighting force. First off the main recruiting area for the new Royal Army was in the south of Italy which had been heavily ravaged by war, and thus the populace was generally unwilling to fight. Secondly, the allies didn't want Italy to develop a strong national army, because a strong army might resist the allied plans at the end of WW2. The allies set a limit of 500,000 men for the army, but by October 1944 the Italian government had only been able to raise about 400,000. The major contribution from former soldiers came in the form of partisan warfare, which hampered the Germans in Northern Italy. There was also internal conflict though. As anti-fascists who had been imprisoned by Mussolini were set free, they began to violently attack former fascists.

Post-war Italy was also in a complete state of chaos. Peasants were starving, and the government exercised very little control, and roving bands of peasants, former guerrillas, and armed thugs hired by land owners roamed the land dealing out their forms of justice. Purges of former fascists were common, especially in the North where officials who had served the fascist government were either arrested, or killed. However, the Badoglio government put a stop to the indiscriminate killings as soon they regained control over the country. In a country like Italy which had a very noticeable North/South split, unity was seen as key in rebuilding the nation. The National Liberation Committee, or the CLN who held control over the North, before Bagdalio's government gained control, held their own little "trials" for the most heinous fascists. Members of the fascist militia, and former black shirts were "tried" and faced harsh penalties. So in the end the Partisans were harsher on war criminals and former fascists the Badoglio government was. Now large amounts of fascists did receive temporary prison sentences, in 1946 there were around 40,000 ex fascists in prison; but this can be both viewed as a punishment and a way to protect them from reprisals.

Sources:

Mussolini by Dennis Mack Smith

The Fall of Mussolini: Italy, Italians, and the Second World War by Phillip Morgan

Italian Foreign Policy in the Interwar Years by H. James Burgwyn

Mussolini Unleashed: 1939-1941 by Macgregor Knox