Radicalism doesn't refer here to the American English sense of the term as a left or right-wing "radical", but to the contrary to the political tradition of Radicalism.
This form of Radicalism, on the whole, has been absorbed by the liberal tradition, although it was during the 19th century a proponent of Republicanism and universal suffrage, and thus often opposed to liberal parties, whom mostly upheld constitutional monarchy and census suffrage.
The most important common feature of these parties is their support of secularism, the defense of the separation of church and state and the promotion of civil rights such as divorce, birth control, homosexuality, gay marriage, abortion, stem-cell research, euthanasia, etc.
In the last few years radical parties in the Netherlands and in Italy came out in support of direct democracy institutions such as the referendum, first-past-the-post electoral system and direct election of prime ministers and mayors.
Therefore, this category includes old (often defunct) Radical parties or parties maintaining a close relationship with Radicalism. See Radicalism and Left-wing politics for further information.