In its original form, Fascism - that is Italian Fascism in its early form of 1919 - was a fringe movement of the radical right, or as historian E. Gentile described it of the "national radicalism" (which works better in context but does not change much of the political collocation), which mixed themes inherited by the national interventionism (that is the non-liberal interventionism, i.e. the right wing interventionism) with certain popular suggestions.
Partecipation to the Great War had in large part reshuffled entirely the political deck in Italy - this had resulted in a transverse split between various groups according to their position on the intervention: liberal interventionists and neutralists, socialist interventionists and neutralists, etc.
This polarization had increased with the major defeat of Caporetto in October-November 1917; when it had assumed more and more the character of neutralist-socialists-defeatist vs. interventionist-nationalist-combatant. It was especially important for the forces of the interventionist field to prove both their committment to the cause of the war and their support for the fighting masses (the good combatants, that is) by pairing a violent press campaign against all the enemies of the Motherland "within and outside" with initiatives in favor of the veterans.
The largest manifestation of this tendency was the formation of a Fascio Parlamentare di Difesa Nazionale - the Parliamentary Fascio of National Defense - roughly equivalent to a political party (fascio was a fairly common name for a political formation at the time, alternative to league or union), in opposition to both the official socialists (that is the neutralists) and Giolitti's Unione Liberale (also neutralists), which comprised a collection of forces ranging from the liberal-conservatives to the Nationalists.
The nature of the political conflict brought to a progressive radicalization within the Fascio that by mid 1918 had resulted in a more divisive political climate and the prevalence in the press and propaganda outlets of certain "more extreme" themes - or, as others have put it, to a victory of radical interventionism over the liberal one.
In this context of post Caporetto radicalization, Mussolini - who had recently come back from the front himself and was already an established personality of the interventionist field - resumed his work as Chief Editor of the Popolo d'Italia on a platform that one could summarize as follows:
Strong support to the war effort and the combatants - combattentismo
The idea that the experience of the trenches had formed a new generation destined to lead the nation through a process of renovation and restoration; a palingenesis driven by and "aristocracy of the trenches" - trincerismo
Growing opposition to socialism, in particular the official Socialist Party (the "Italian Bolsheviks" as the national press had begun to call them) accused of defeatism - culminating in the choice to remove the subtitle "socialist newspaper" from the headline of the Popolo d'Italia and, as Mussolini explained, "to close that shop for good and leave the market to the competition [...]" since socialism was "thrift store junk from before the war"
Opposition or at least a strong subordination of democratic values to national values
After the War ended, Mussolini attempted to continue his activity retaining the elements of veteranism, combatantism and anti-socialism; but without committing to any specific political formation or program at least until early 1920.
The first fascio properly speaking - that formed in Milan on March 23^rd 1919 under the name of Fascio Italiano di Combattimento (Italian Fascio of the Combatants) was one of the many associations risen from the experience of radical interventionism, with the purpose of appealing to the veterans returning from the war - it was created in parallel with the action of other at the time more fortunate initiatives such as D'Annunzio's Legionari who were in the process of attempting a coup to seize the contested city of Fiume, and the famous assault squads, the Arditi who were trying to evolve into a political formation as well.
All those formations failed more or less spectacularly - the fasci counted 870 members nationwide by the end of 1919 and failed to win any election (national or local) even when running with other combatants formations.
It was the agrarian (violent) reaction to the rise of the socialist leagues during 1920-21 that led many of the former arditi, legionnaires, veterans, to form cores over which a part of the agrarian masses inspired by anti-socialist sentiments and partly driven by economic needs could coalesce, breathing new life into a movement that had appeared on the brink of death.
By the end of 1920 the fasci had 20,615 members; 249,036 by the end of 1921.
At this point Mussolini was fully committed to make fascism his political creature and to secure a stronger control on the organization; thus resulting in the formation of the Fascist National Party on November 7^th 1921.
Obviously, the agrarian forces had not funded the fasci (and largely so) without exerting some form of ideological influence; which contributed to fascism losing some of its intial "radical" undertones - such as the revendication (typical of radical interventionism) for a distribution of "the land" to the veterans - and becoming more conservative on themes such as the church and the monarchy.
So, when the PNF made its entrance into the political scene, it was a national-conservative force, with strong anti-socialist positions (and a consolidated practice of violence against political oppositions), a fairly generic but no longer too extreme program; which retained much of its imagery and myths mutuated from the field of national radicalism - from the idea of a necessary renovation of the state and individual alike, to the cult of energy, will and (violent) action.
This mishmash of ideas persisted within the fascist movement well into its affirmation as the dominant political force in Italy - until a certain belated systematization was attempted by Mussolini himself (with the notorious "Doctrine of fascism") in the 1930s. But the syncretic nature and the ability of Fascism to digest certain aspects of other political forms when needed - as well as to compromise and adapt to the circumstances surrounding it, such as its long coexistance with the Church and the King - remained decisive factors in its "definition".
For this reason it gets less and less convenient to attempt an ideological definition of Fascism, with its progressive evolution and consolidation of power - historians of Fascism like R. De Felice had attempted a solution in positing the presence of a "Fascism-Party" and a "Fascism-Movement" and looking at such evolution through the dialectics between those two instances. There's not one and only correct way to follow the evolution of Fascism, though; but it is only possible to abstract from its real ongoing progression in so far as one already understands it.
So, for the purpose of this answer, I don't think I can attempt to trace the evolution of fascism unless I follow also the whole political evolution of the Regime - which I can't.
I would recommend checking out R. Paxton's Anatomy of Fascism - also works on comparative interpretations of Fascism have been provided by R. De Felice and more recently by E. Gentile.
If you are interested in the evolution of "fascism" as a concept, political identifier, etc. after 1945; I fear I can't give you much. Z. Sternhell's works (especially The birth of fascist ideology) provide an interesting connection between early day fascism and present day fascism - a view that is not uncontroversial and that I am not sold on - but might be of some interest for your question.