"Let us kneel before the Lord who made us"
1) 'was wondering' asked: “Why do we use the host instead of real bread for the body of Christ?”
The following comes from Wikipedia; to view the full text and to see photos of hosts and the instruments used to make them, please click on the title of this post.
A host is a thin, round wafer made from bread and used for Holy Communion in many Christian churches.
The word is from the Latin, "hostia", which means "victim" or "sacrificial animal." The term can be used to the bread both before and after consecration, though it is more correct to use it after consecration - "altar bread" being preferred before consecration.
In the Roman Catholic Church, hosts are often made by nuns as a means of supporting themselves. It is required that the hosts be made from wheat flour and water only (Code of Canon Law, Canon 924). The Roman Church teaches that at the words of consecration the bread is changed into the Body of Christ through transubstantiation...
2) 'Anon': "Why, at some churches, does the host look different?"
If you look at the photos from the wikipedia site, you’ll see that there are different “tongs” within each convent which make the hosts look different. Also, hosts are made at multiple convents, so there are multiple tongs / designs of hosts.
3) 'Short hair': "Why is it that it’s only in the Catholic churches that people kneel?"
I don’t know if this is definitely true or not, but if it is, I would guess it’s because the Catholic Church takes transubstantiation so seriously that she requires her members to kneel in the presence of the Lord, out of reverence for Jesus in the Eucharist. One biblical basis for this is Psalm 95: “let us kneel before the Lord who made us”.