Ordinary Time begins following the Baptism of the Lord, which in the United States is on the Monday after the Sunday on which the Epiphany is celebrated, unless the diocese celebrates the Epiphany on the traditional January 6 date, in which case the The Baptism of the Lord is celebrated on the Sunday after Epiphany and Ordinary Time then starts on that Monday. It continues until Ash Wednesday.
The term Ordinary Time corresponds to the Latin Tempus per annum (literally "time through the year"). The term Ordinary does not mean common or plain, but is derived from the term ordinal or "numbered." Sundays are then designated in the style First Sunday of Ordinary Time, Second Sunday of Ordinary Time, etc. Several Sundays, such as the Trinity Sunday or the Feast of Christ the King are part of Ordinary Time, but have names because they are Feasts.
Even though Ordinary Time is divided into two periods, one after the Christmas Season and one after the Easter Season the Sundays are number sequential. So if there are seven weeks of Ordinary Time before Lent, as there is this year, the Eighth Sunday of Ordinary Time will be after the Easter Season. Last year there were eight weeks of Ordinary Time between the Christmas season and Lent, so the Eighth Sunday of Ordinary Time was the Sunday before Ash Wednesday.
Green is the Liturgical color for Ordinary Time, though other colors are worn for specific Feasts and Solemnities during Ordinary Time, typically white.