What is included in a patent application, in the US and EU?

A patent application has four core components in both the US and EU: the specification, a full written description of the invention; the claims, the precise legal boundaries of what's protected; drawings, technical figures where necessary to understand the invention; and an abstract, a short summary. Claims are the part that actually gets enforced, so founders who over invest in the specification and under invest in claims are protecting the wrong thing in either jurisdiction. The EU adds one requirement the US doesn't have: you must specify which of the EPO's contracting states you want the eventual patent to cover, and translate the claims into two further EPO official languages (English, French, and German) before grant.

Sources: USPTO, Nonprovisional Patent Application Basics, EPC Article 78, Requirements of a European patent application.