A Teisho is a talk by a sanctioned teacher, usually given during an intensive meditation retreat known as a sesshin. The teacher faces the Buddha altar during a Teisho. Typically, the teacher will sit on a raised platform.
A Dharma Talk is a talk by a senior student who has not received sanction to teach. The speaker faces the sangha during the talk and sits on his or her normal cushions, there being no raised platform in use. Dharma talks are typically given during regular meetings of the meditation group and usually supplant a round of meditation.
Although Zen practitioners sit without motion during periods of formal zazen, most Zen centers have a rule that allows the listeners to adjust their posture during a teisho or a dharma talk because listening is the paramount activity during a teisho or a dharma talk.