In a smaller system, design of the entire system can be shown in one place.
This class diagram of se-edu/addressbook-level3 depicts the design of the entire software.
Design of bigger systems needs to be done/shown at multiple levels.
This architecture diagram of se-edu/addressbook-level4 depicts the high-level design of the software.
Here are examples of lower level designs of some components of the same software:
Multi-level design can be done in a top-down manner, bottom-up manner, or as a mix.
Top-down design is better than bottom-up design.
Explanation: Not necessarily. It depends on the situation. Bottom-up design may be preferable when there are lot of existing components we want to reuse.
Agile design can be contrasted with full upfront design in the following way:
Agile designs are emergent, they’re not defined up front. Your overall system design will emerge over time, evolving to fulfill new requirements and take advantage of new technologies as appropriate. Although you will often do some initial architectural modeling at the very beginning of a project, this will be just enough to get your team going. This approach does not produce a fully documented set of models in place before you may begin coding. -- adapted from agilemodeling.com
Agile design camp expects the design to change over the product’s lifetime.
Explanation: Yes, that is why they do not believe in spending too much time creating a detailed and full design at the very beginning. However, the architecture is expected to remain relatively stable even in the agile design approach.