Web Farm and Web Garden : A Web farm allows you to expand a Web Site across multiple servers. A Web garden allows you to expand a Web Site across multiple CPUs within a single server.
Alternate Access Mapping: AAM is way to define the different URLs associated with Web Application and it's set of content database. It allow to map internal and external URLs that users see. E.g. You may have two different web for customer and internal user. You may map alternate access such that user feel like working with same portal but in different part of portal having variation in URLs they see.
reference : MSDN