I'll give this a shot for ya!
I'll start with the schema. The schema defines how objects are created in AD. When you create a user in the directory for example it looks at the schema to determine what properties make up that user object. It looks at the schema to determine what to put on the general tab or the account tab of that users property sheet. So the schema holds all of the definitions for all objects. Some applications, such as exhcange server, actually extend the schema to add more object types as well as add more properties to existing objects. There can be only one schema defined in a forest and one of your domain controllers is your schema master, usually the first one you install.. though you can move roles around. Here's a site with some more schema info:
http://msdn.microsoft.com/en-us/library/ms675085(VS.85).aspx
Organizational units are logical containers used to organize objects in AD. For example, if you have a sales department and a marketing department you could create a sales OU and a marketing OU and put all of the sales users in sales and the marketing users in marketing. This has multiple benifits, one being that for a large organization it's easier to keep track of objects and another is group policy. You can assign policies to a specific OU. So if you don't want anyone in sales to have access to the control panel you could do that without affecting marketing users.
The Global Catalog is a special server that contains a partial replica for all objects in a forest. A forest is an environment with more than one domain. The global catalog is important during the logon process because it also holds universal group membership. If a domain controller can't communicate with a global catalog to determine this information during logon the authentication request will be denied. By default the first domain controller installed in each domain is a global catalog server. You can add more in Active Directory Sites and Services. Here's some more on the Global Catalog as well:
http://technet2.microsoft.com/windowsserver/en/library/24311c41-d2a1-4e72-a54f-150483fa885a1033.mspx?mfr=true
The LDAP protocol is what is used to query the ad database. It's also used by Netware's eDirectory and most other directory services out there.
It's not possible to cover everything about AD here. I didn't even touch on things such as the partitions that make up AD and replication between domain controllers. I've been a MCT for a few years now and i've taught many AD classes and it's a pretty deep subject. I do hope this at least gets you in the right direction!
Good luck