The two most important devices used in Gibran's "Song of the Rain" are metaphor and personification.
The first two stanzas begin, respectively, with the phrases "I am dotted silver threads dropped from heaven," and "I am beautiful pearls." The poet does not mean this phrases literally; rather he means that the rain is like silver threads from heaven and beautiful pearls. Such comparisons that do not use the words "like" or "as" are called metaphors.
Gibran also makes extensive use of personification, which means that he speaks about inanimate objects--such as the rain and fields--as if they were human. Examine, for example, stanza 3:
When I cry, the hills laugh;
When I humble myself the flowers rejoice;
When I bow, all things are elated.
Stanzas 4-8 also contain many examples of personification.
The poem also contains two similes--comparisons that do use the words "like" or "as." The rain is "like earthly like," and it "kills" the heat in the air "as woman overcomes man with / The strength she takes from him."
"Song of the Rain" also contains an interesting reference to another work of art; this is known as an allusion. Stanza 2 reads:
I am beautiful pearls, plucked from the
Crown of Ishtar...
To embellish the gardens.
Ishtar was the ancient Middle Eastern goddess of love and fertility (among other things). By comparing the rain to "pearls plucked from the Crown of Ishtar," the poet introduces the themes of rain as the great source of plant fertility, and also the theme of rain as a symbol of love. This second theme--of rain's connection to love--is discussed in stanzas 3, 6, and 7.