I've read Wikipedia's definitions for these:
But I can't really discriminate between the two.
Is the difference about the revolt of the workers in Collectivism and also possibly violence to achieve the aim?
But then the Syndicalism article talks about "direct action", which also seems to be a revolt (which could also be violent).
Both those Wikipedia articles reference Revolutionary Catalonia but I can't tease out the differences.
The two epithets are essentially orthogonal:
The "-syndicalism" suffix regards the tactical or strategic choice of labor-based, workplace-based, organizational forms as the main vehicle for overthrowing the established social order and transforming society. These are revolutionary unions or syndicates.
"Collectivist" Anarchism regards key aspects of the envisioned post-revolutionary, non-statist economic system. In collectivism, workplaces are managed by the workers collectively; each worker receives "labor notes", a substitute for money, based on amount worked and job difficulty, as determined by democratic institutions (i.e. not by some manager or owner).
Collectivism differs from Mutualism, which has more money-like means of exchange, as well as banking and other features; and on the other hand, from Communism (not the USSR-style, the original definition) in which distribution is based on need and/or desire, not on amount worked, and typically there is no exchange of goods or tokens-of-value like the collectivists' labor notes.
More on Mutualism, Collectivism and Communism: The Economics of Anarchy (don't worry, it's not a book, just a web article).
Having said that - there is a tendency to identify the post-revolutionary outlook of Anarcho-syndicalists as essentially collectivist. If you subscribe to this view, then you would think of Anarcho-Syndicalism as "Collectivism with a focus on unions/syndicates/the labor movement as a strategy" (this is essentially the description in An Anarchist FAQ Section A.3 for example).
Personally I disagree with this view, and find that one can very well focus on workers' organization in current society while opposing both currency and labor notes in a post-capitalist society.
Both of those definitions have been somewhat fluid throughout time due to the change in context. I would argue that Collectivist Anarchism is a school of though with a well known origin and supporters. More of an academic endeavor (although experiments were made) than Anarcho-Syndicalism whose nature rooted in direct action make it more of a militant movement.
Historical Background
Collectivist Anarchism represents a specific school of thought founded by Mikhail Bakunin (1814-1876).
Bakunin's socialism was known as "collectivist anarchism", where "socially: it seeks the confirmation of political equality by economic equality. This is not the removal of natural individual differences, but equality in the social rights of every individual from birth; in particular, equal means of subsistence, support, education, and opportunity for every child, boy or girl, until maturity, and equal resources and facilities in adulthood to create his own well-being by his own labor."
Collectivist anarchism advocates the abolition of both the state and private ownership of the means of production. It instead envisions the means of production being owned collectively and controlled and managed by the producers themselves. For the collectivization of the means of production, it was originally envisaged that workers will revolt and forcibly collectivize the means of production. Once collectivization takes place, money would be abolished to be replaced with labour notes and workers' salaries would be determined in democratic organizations based on job difficulty and the amount of time they contributed to production. These salaries would be used to purchase goods in a communal market.
It's important to notice that Bakunin Collectivism was proposed at a time when major schools of thought for Socialism (i.e. social ownership of the means of production) appeared. Bakunin was one of the most well known critiques of Marxism (see 1st International for more on this).
Anarcho-Syndicalism on the other hand has a more fuzzy origin strongly associated with notion of syndicalism. As Rudolph Rocker put it:
Anarcho-syndicalists believe that workers' organisations that oppose the wage system will eventually form the basis of a new society and should be self-managing. They should not have bosses or "business agents"; rather, the workers alone should decide on that which affects them.
To this day Anarcho-Syndicalism is still very much associated with labour associations, notably the spanish CNT-AIT.
Comparison
I always feel a bit hesitant when comparing two philosophies separated by time, context, and place, because a very objective, listed point by point, difference chart tends to lead to the conclusion that both terms are definite, strict things. They are not. You ask two persons, you get two answers. And I doubt you'll ever find written on stone rules for something like Anarcho-Syndicalism (for more refer to this source).
Syndicalism, like communism, collectivism, and mutualism, is one of the major trends for Social Anarchism. They mostly have different strategies with very similar objectives.
Anarcho-syndicalists, like other syndicalists, want to create an industrial union movement based on anarchist ideas. Therefore they advocate decentralised, federated unions that use direct action to get reforms under capitalism until they are strong enough to overthrow it. In many ways anarcho-syndicalism can be considered as a new version of collectivist-anarchism, which also stressed the importance of anarchists working within the labour movement and creating unions which prefigure the future free society.
However, unlike other major trends, it does not have philosophers who founded its principles. It is instead a product of the context of the time:
Syndicalism is somewhat different, as it was far more the product of workers' in struggle than the work of a "famous" name (although this does not stop academics calling George Sorel the father of syndicalism, even though he wrote about a syndicalist movement that already existed).
So it is actually possible that people identifying themselves as Anarcho-Syndicalists actually be practicing one of the other three (or others) schools of thought.
Collectivist anarchist is an anarchist ideology that proposes a model for the organization of society.
Anarcho-syndicalism is a method used to advance towards an anarchist society, by using trade unions as a means. Basically it consist of anarchist organizing themselves in trade unions. Anarchist trade unions are useful because:
they serve as a platform to reach to workers; the workers at your workplace maybe will not care to join the "anarchist gardening club" or the "anarchist role playing gaming club", but a trade union that is active in defending the workers'rights will get more attention.
they tie together the issues of workers'rights and revolutionary action; people fighting for workers'right may chose to support an ideology to supports revolutionary change. Specially if just promoting the workers'right already puts you in risk of being harassed by the police, beaten or murdered.
they are useful as a platform of action against capitalists, whatever by "regular" trade union methods (organizing striking and public protests) or more "revolutionary" methods (occupying factories and farms and attempting to take them over), to showcase the willingness of the anarchist to promote their fight, as a form of Propaganda of the deed.
