2. Ordinary Magisterium: this second form of Church teaching is “continually exercised by the Church especially in her universal practices connected with faith and morals, in the unanimous consent of the Fathers and theologians, in the decisions of the Roman Congregations concerning faith and morals, in the common sense of the Faithful, and various historical docuмents, in which the faith is declared.” (Definition from “A Catholic Dictionary”, 1951)
So, according to this definition, the Ordinary Magisterium (also referred to as the Universal Ordinary Magisterium) is Church teaching that is continuous and unanimously consented to throughout the Church.
"A Commentary on Canon Law" (Augustine, 1918, Canon 1323, pg 327) states: "The universal and ordinary magisterium consists of the entire episcopate, according to the constitution and order defined by Christ, i.e., all the bishops of the universal Church, dependently on the Roman Pontiff". It also states, "What the universal and approved practice and discipline proposes as connected with faith and morals must be believed. And what the Holy Fathers and the theologians hold unanimously as a matter of faith and morals, is also de fide."
The Ordinary Magisterium is where the majority of Catholic beliefs are taught and learned; through unanimous teaching by preaching, by any written means, the approval of catechisms, the approval of textbooks for use in seminaries, etc.
Some examples of the Ordinary Magisterium would be that of Guardian Angels, or the Assumption of the Blessed Virgin Mary (before 1950). While neither were solemnly defined by the Church (before 1950), they were always universally taught and believed, and it would be considered heresy to deny them.
For example, Arius was considered a heretic before his condemnation at the Council of Nicaea in 325, because the Divinity of Christ (which he denied) was part of the teaching of the Ordinary Magisterium before that Council. The same applies to Nestorius regarding his denial of the Divine Maternity of the Blessed Virgin, where he was later declared a heretic by the Solemn Magisterium at the Council of Ephesus.
So in a nutshell, the Solemn Magisterium (used rarely) plus the Ordinary Magisterium (used continuously) equals the complete infallible teaching of the Catholic Church. The article "Science and the Church" from the Catholic Encyclopedia (1917) states it well: "The official activity of teaching may be exercised either in the ordinary, or daily, magisterium, or by occasional solemn decisions. The former goes on uninterruptedly; the latter are called forth in times of great danger, especially of growing heresies."
Finally, the most frequent reason why the Solemn Magisterium is used is in order to confirm a doctrine which already belongs to the Ordinary Magisterium, but which has come under attack, usually by heretics.