Too must questions for 'small' subject.
In early Httpd 2.2, only 'prefork' (process-based) and 'worker' (thread-based; share-based memory; better for multiple-core/processor). Since 2.2.8, 'event' (variant of worker; with concurrency) also as stable (rather than experimental) like worker. On httpd 2.4, event as default mpm.