What are the similarities between Roman Catholicism and Catholicism?

Catholic
from A Catholic Dictionary, edited by Donald Attwater, Second edition, revised 1957
I. The word is derived from Greek and simply means universal. In combination with the word "church" it essentially merely indicates one of the marks of the Church, and was so used by St. Ignatius at the beginning of the 2^nd century; but in the course of history it has come to be the distinguishing epithet of the Church of Christ and his faith: under other circumstances its place might have been taken by "apostolic" or "one." The use of the word in this distinguishing way became current and common in England only from the middle of the 16^th century. In some mediaeval translations of the Creed unam sanctam catholicam et apostolicam ecclesiam is rendered "one holy apostolic church general."
ii. A Catholic is any person who, having been baptized, does not adhere to a non-Catholic religion or perform any act with the intention or effect of excluding himself from the Church. A "good Catholic" is one who practises his religion to the best of his ability.
iii. Catholics normally call themselves Catholics without qualification, and are distinguished by the name alike in West and East; except for a body of High Anglicans, no other Christians use the name as a distinguishing title. But Catholics of the Byzantine rite sometimes calls themselves Greek Catholics, Chaldeans are so called, and Maronites always refer to themselves simply as Maronites - they avoid the name Catholic for the good reason that there is no such thing as a Maronite who is not a Catholic, and because in Syria the epithet particularly designates a Catholic Melkite.
iv. As an adjective, Catholic in this special sense should only be used of subjects of which Catholicity is predicable, e.g., a man as man, a church, building, or catechism. To speak of a Catholic artist or grocer, Catholic poetry or truth is inaccurate and misleading: an artist or grocer who is a Catholic is a Catholic as a man (and this without reference to whether he paints only ecclesiastical pictures or supplies cheese only to the clergy); poetry may deal with a Catholic theme or be written by a poet who is a Catholic, but is not by that fact anything but poetry; truth is truth and it is improper to call the truth about the Catholic Church, Catholic truth; (cf., Catholic arithmetic, a Wesleyan judge, Quaker music, and, particularly, Catholic culture).

Roman Catholic
A name used by many English-speaking non-Catholics for members of the Catholic Church, as a qualification of their exclusive right to be called Catholic, of whatever rite, looks to Rome as the centre of the Church and the seat of her supreme pontiff and head, the expression in itself is unobjectionable and is in fact sometimes employed by them, especially in certain countries of Europe. But its use by Catholics is unnecessary and, having regard to its connotation for many non-Catholics, sometimes to be avoided.