Casually, they mean the same thing: the place from which readers read, cantors chant, and preachers preach. The original term for the whole thing was the Greek word ambo. When “church” evolved from being a name for the assembly to designate the special building where people gathered, architecture began to define the liturgical movements. Because the Mass comes in two parts, the Liturgy of the Word and of the Eucharist, the ambo was the place where the first part happened, and the altar was the stage for the second.
We see the vestige of the ambo design in the semicircular part of the sanctuary that juts into the assembly. The ambo pulpit was first positioned there on an elevated platform. Two staircases led to it. The subdeacon ascended from the east and, facing the altar, proclaimed the epistle. The deacon ascended from the west, facing the people, and proclaimed the gospel. Because both readings were chanted, this front area also housed the choir and was part of what today would be called the “music ministry.” Preaching was normally done from the presider’s chair.
The ambo design imitated the mountain where Moses received the Law and Jesus offered his famous Sermon. From the 4th-12th centuries this configuration was popular, leading to developments such as two ambos: an eastern one dedicated to the epistle and a western one with a permanent candle used for the gospel. Less common was the double-decker ambo with a lower station for the epistle and higher one for the gospel.
The pulpit eventually replaced the old ambo. Less ornate in decoration, it was still elevated (pulpit, by the way, means “scaffold”). The pulpit was separated from the choir and used purely for proclamation, its exalted stage viewed as the "position of the perfect.” Even during the early ambo period, acoustics were poor from the chair so some sermons were delivered from the ambo. The pulpit supported this tradition and is now usually the name for the place from which priests and deacons read the gospel and give the homily.
The lectern is a humbler development: It’s a support for a book. It may denote the stand the priest uses to prop up the sacramentary at the altar. Today, ambo and lectern are often used interchangeably to refer to the place where the readings, psalm responses, and general intercessions are proclaimed. The pulpit is generally reserved for preaching and the gospel reading.
• Built of Living Stones: Art, Architecture, and Worship, United States Conference of Catholic Bishops, Committee on Divine Worship
• The House of God: Church Architecture, Style and History by Edward R. Norman (Norton/Thames and Hudson, 2005)
• Repitching the Tent: Reordering the Church Building for Worship and Mission by Richard Giles (Liturgical Press, 1999)