Thanks for your question; it's an issue about which many people are curious.
As we begin our brief examination of angels, we must remember to never go beyond what God has laid out for us in Scripture. There is much bad information about angels circulating today because people are offering conjectures rather than quoting Scripture about angels.
It's important to note that angels are created beings (Colossians 1:16; Psalm 148:2,5) yet do not have flesh-and-bone bodies (cf. the use of the term "spirits" to describe demons in Matthew 8:16 and 12:45). They do not marry (Matthew 22:30). In this sense they are considerably different from human beings. Yet they are superior to humans in knowledge (Matthew 24:36), in glory (Psalm 8:5) and in the ability to worship and, now, to serve God without hindrance (Revelation 5:11).
What role(s) do angels serve? Scripturally, there is no support for the notion that each Christian has a "guardian angel." We do know that the Lord utilized angels as His special messengers before revelation ceased with the Book of Revelation (cf. St. Luke 2:9, etc.). They are our exemplars when it comes to worshipping God purely and vigorously. They also, at one point in salvation history, fought for God's people in executing His perfect will (cf. Exodus 33:2).
As far as rank within the angelic host (and "host" generally means "a great number"), Scripture identifies Cherubim (Genesis 3:24, Exodus 25:18 etc.), Seraphim (Isaiah 6:2,6), "principalities and powers" (Ephesians 3:10), and by name mentions Gabriel (Luke 1:19,26) and Michael (Daniel 10:13,21 and Jude 9, etc.). That the angels are organized reflects the wisdom and orderliness of God (cf. I Corinthians 14:40).
Did angels suffer from original sin? They did not suffer from it in the sense that the natural world and humanity have felt its effects, but the Scriptures teach little in regard to this topic other than to say the Lord pronounced His creation "good" prior to the fall. Some of them obviously fell with Satan and now work to oppose Christ and his kingdom (Luke 11:17-21 etc.), which was a matter of angelic volition (or choice). The angels of light, who chose to do the will of God, apparently shall persevere in that state of glory (Luke 20:36). Those who followed Satan shall suffer eternal punishment (II Peter 2:4), and there is no hope of redemption for them.
In summary, what may we learn from the existence and from the work of the angelic host? We learn to worship and to serve the Lord who loved us and gave himself for us, even as do His angels in heaven (Matthew 6:10).