Let me start with the term "line." A line in a poem is simply that: a line of words, which are moving from the left to the right, without going further up or down within the poem. An example of this can be seen in the poem "Planetarium" by Adrienne Rich:

A woman in the shape of a monster
a monster in the shape of a woman
the skies are full of them

Here, "A woman in the shape of a monster" is a line", "a monster in the shape of a woman" is another line, and so is "the skies are full of them."

A stanza is several lines put together to make a whole unit. A stanza is usually a unit which contains the meter of the poem. A stanza can be recognized visually, as there usually is a gap between stanzas to indicate the end of one stanza and the beginning of the next. In fact, the example used above is also an example of a stanza, as all three lines together form a stanza. Here it is again, this time also with the second stanza:

A woman in the shape of a monster
a monster in the shape of a woman
the skies are full of them
a woman ‘in the snow
among the Clocks and instruments
or measuring the ground with poles’