They both mean "in addition", but their placements vary, and with them, what they apply to. One obvious difference is that only "also" can start a sentence: "Bob ate an apple. Also, he ate an orange." (But it would mean the same and be more natural here to say "He also ate an orange".) "Too" usually goes to the end of the sentence: "He ate an orange too". It doesn't have to, and one notable exception is when you want it to apply to the subject rather than the object: "I too ate an orange". "I also ate an orange" is different -- the "also" still applies to the orange.