Typically, RUP is presented as a use-case driven, architecture first, iterative-incremental process divided in four phases: inception, elaboration, construction, and transition.
Somehow, using a manufacturing metaphor (I know, I know... usually are the worst ones, and can't go too far), the project can be divided into 2 major stages: "engineering" (inception and elaboration) and "production" (construction and transition).
The good thing is "engineering" aims to reduce significantly the risk level of the project. The biggest uncertainties at the beginning of a project are usually related to requirements (and scope) or technical ones, and they have a big impact because they’re the major sources of cost and time overruns. The traditional approach like the “waterfall” process is to pinpoint all the requirements and then complete the design to the detail (what the “agilists” call “Big Requirements Up Front” and “Big Design Up Front”). I will not beat the “Waterfall” straw man, it has its niche (albeit a lot smaller than most people think). Other option is ignore them and hope that the team is “agile” enough to respond to the risks when they materialize.
In the case of RUP, there’s an explicit intention to “defuse” those risks, by exposing and validating the assumptions as soon as possible:
- the inception phase main goal is to discover what it should be done (usually identifying most of the use cases, and detailing the most critical ones)
- the elaboration phase main goal is to define how it should be done, by defining the architecture of the system (based on the critical use cases)