Fascism is a political ideology, imperialism is a diplomatic mode of action. Not all fascist regimes acted imperialistically (even though most of them did, and all of them probably wanted to), and not all nations who acted imperialistically were fascist.
Imperialism is rather straightforward (unless you're a Leninist, I suppose), and most dictionaries can give you satisfying definitions.
State policy, practice, or advocacy of extending power and dominion, especially by direct territorial acquisition or by gaining political and economic control of other areas.
The policy, practice, or advocacy of extending the power and dominion of a nation especially by direct territorial acquisitions or by gaining indirect control over the political or economic life of other areas.
The policy of extending a state's influence over other peoples or territories.
Let's define fascism then, which is a bit more diverse in its interpretations by historians. There are numerous popular definition, the most popular one perhaps that given by Italian author Umberto Eco in "Ur-Fascism", but I am quite partial towards the definition given by historian of fascism Stanley G. Payne in his "History of Fascism 1914-1945", which goes as follows:
A. IDEOLOGY AND GOALS
B. THE FASCIST NEGATIONS
C. STYLE AND ORGANIZATION
In my opinion, this is the most satisfying definition of "fascism" as a term (even though I'm fully aware that this fits my personal biases, and there is no inherent reason why this definition is any more or less correct than any other sociologically derived definition). So let's run with it.
The specific subpoint that will be of the most immediate interest to your question is A.5, the "goal of empire". Indeed, all the major fascist nations that we think of (in fact, Payne only recognizes Nazi Germany and Fascist Italy as fully valid examples of fascism in history) had the goal of empire, be that Nazi Germany's ambition to acquire "Lebensraum im Osten" and to enforce "Generalplan Ost", Fascist Italy's intent to secure "spazio vitale", or Japanese (if we accept that Japan was fascist, which Payne rejects) concerns about overpopulation in the Japanese archipelago and its ambitions to resettle Japanese colonists in Korea and China.
So it is almost undeniable that all the major fascisms were imperialistic, but that does not make the two terms definitionally equivalent. First of all, there were imperialistic powers that clearly weren't fascistic, be that the liberal democratic monarchical United Kingdom, the liberal democratic republican France, or the liberal democratic republican United States. There were also systems that you could define as "ethnically imperialistic democracies", most notably the Apartheid states of South Africa and Rhodesia. Both of these systems were multi-party democracies and as such clearly not fascistic under Payne's definition, but by denying the franchise to their ethnic majorities and by occupying said ethnic majorities' living areas and exploiting them, they very clearly were behaving under ways that we could define as imperialist.
Additionally, if we extend beyond Payne's very strict focus on Italy and Germany, there were fascist states that weren't notably imperialistic. Austria's Ständestaat of the mid-1930s has often been called fascist (or "austrofascist") before its Anschluss by Nazi Germany, but beyond its diplomatic intent to overturn the Treaty of St Germain, it did not commit any major acts of "imperialism".
So, yeah, I guess that's the difference: These two are completely separate concepts from two separate families of political terminology (ideology versus diplomacy). But there are some overlaps: imperialism was often included in fascism, even though fascism was not always included in imperialism.