Although secularism and secularization are closely related, there are real differences because they do not necessarily offer the same answer to the question of the role of religion in society. Secularism is a system or ideology based on the principle that there should be a sphere of knowledge, values, and action that is independent of religious authority, but it does not necessarily exclude religion from having any role in political and social affairs. Secularization, however, is a process which does lead to exclusion.
Process of Secularization
During the process of secularization, institutions throughout society - economic, political, and social - are removed from the control of religion. At times in the past this control exercised by religion might have been direct, with ecclesiastical authorities also having authority over the operation of these institutions - for example, when priests are in charge of the nation's only school system. Other times, the control might have been indirect, with religious principles constituting the basis for how things are run, such as when religion is used to define citizenship.
Whatever the case may be, either those institutions are simply taken away from religious authorities and handed over to political leaders, or competing alternatives are created alongside the religious institutions. The independence of these institutions in turn allow individuals themselves to be more independent of ecclesiastical authorities - no longer are they required to submit to religious leaders outside of the confines of a church or temple.
Secularization & Church/State Separation
A practical consequence of secularization is the separation of church and state - in fact, the two are so closely associated that they are almost interchangeable in practice, with people often using the phrase "separation of church and state" rather when they mean secularization. There is a difference between the two, though, because secularization is actually a process that occurs across all of society, whereas the separation of church and state is simply a description of what occurs in the political sphere.
What the separation of church and state means in the process of secularization is that specifically political institutions - those associated with varying levels of public government and administration - are removed from both direct and indirect religious control. It does not mean that religious organizations cannot have anything to say about public and political issues, but it does mean that those views cannot be imposed upon the public, nor can they be used as the sole basis for public policy. The government must, in effect, be as neutral as possible with respect to divergent and incompatible religious beliefs, neither hindering nor advancing any of them.
Religious Objections to Secularization
Although it is possible for the process of secularization to proceed smoothly and peacefully, in reality that has often not been the case. History has shown that ecclesiastical authorities who have wielded temporal power have not readily handed over that power to local governments, especially when those authorities have been closely associated with conservative political forces. As a consequence, secularization has often accompanied political revolutions. Church and state were separated in France after a violent revolution; in America the separation proceeded more smoothly, but nevertheless only after a revolution and creation of a new government.
Of course, secularism has not always been so neutral in its intent. At no point is it necessarily anti-religious, but secularism does frequently promote and encourage the process of secularization itself. A person becomes a secularist at the very least because he believes in the need for a secular sphere alongside the religious sphere, but more likely than not he also believes in the superiority of the secular sphere, at least when it comes to certain social issues.
Thus, the difference between secularism and secularization is that secularism is more of a philosophical position about the way things should be, while secularization is the effort to implement that philosophy - even sometimes with force. Religious institutions may continue to voice opinions about public matters, but their actual authority and power are restricted entirely to the private domain: people who conform their behavior to the values of those religious institutions do so voluntarily, with neither encouragement nor discouragement emanating from the state.