"The Tyger" and "The Lamb" are two poems which Blake published in his poetic collections Songs of Innocence (1789) and Songs of Experience (1794). "Tyger" is actually an archaic spelling of the word "tiger"; it was quite popular with many old scholars and writers until the 1800s, when "tiger" became the official spelling of the word. I think Blake wrote "tyger" instead of "tiger" on purpose, because the tyger in the eponymous poem is not actually a tiger, but a metaphor for everything that opposes nature, serenity, and harmony. "The Tyger" in this case is the polar opposite of "The Lamb," which symbolizes everything pure, innocent, and natural. Some analysts even believe that the tyger is actually a symbol of the industrialized society that slowly started to destroy and pollute the natural environment.