Parts of ISO 9241 that are interesting to designers

ISO 9241 is practically a large pile of documents where usability standards are stored. It's also known as “The Ergonomics of Human System Interaction”.

It is considered important because of its sheer amount of information on usability. Expect it to cover hardware, software and processes. Several countries (especially in Europe) have even adopted some of these standards in their national law.

What's in it for UX designers?

Few people would be interested in all the parts of the ISO 9241. In the following list, I will outline several of these documents that might interest UX designers. (If I have missed one very important part, please let me know in the comments below.)
Part 11: Usability: Definitions and concepts

Gives a framework for understanding the concept of usability and applying it to situations where people use interactive systems, and other types of systems and products and services.

Part 100: Introduction to standards related to software ergonomics

Gives a brief overview of the content of the standards, what the relationship is between several standards and helps understand several definitions of Usability.

Part 110: Dialogue principles

Presents usability questions that apply to dialogues (the most used link between people and information systems). It presents the seven principles of these dialogues:

1. The suitability for the task: The dialogue should be suitable for the task.
2. Self-descriptiveness: the dialogue should make clear what the user should do next.
3. Controllability: the user should be able to control the pace and sequence of the interaction.
4. Conformity with user expectations: it should be consistent, also with other established interactions.
5. Error tolerance: the dialogue should be forgiving.
6. Suitability for individualisation: the dialogue should be able to be customised to suit the user.
7. Suitability for learning. The dialogue should support learning.

Part 112: Principles for the presentation of information

Gives design principles related to the presentation of information by user interfaces. It helps UX designers understand and design to improve the perception and understanding of information (either visual, auditory, or tactile). It is really focused on helping the user to complete a task (not to corporate branding and advertising).

Part 125: Guidance on the visual presentation of information

Guides the reader throughout a visual design process and the basis of an evaluation. It gives recommendations on syntactic or semantic aspects of information and gives pointers on the organisation of information.

Part 129: Guidance on software individualisation

This part provides guidance on how the various aspects of individualisation are made usable and accessible, but also where it might be appropriate or inappropriate.

Part 151: Guidance on World Wide Web user interfaces

Gives guidance on the human-centred design of web user interfaces with the aim of increasing usability, with the focus on design strategy, content design, navigation and search; and content presentation.

Part 143: Forms

Gives requirements and recommendations for the design of forms (e.g. contact forms or payment forms), in which the user fills-in or selects entries.

Part 161: Guidance on visual user interface elements

This part describes visual user-interface elements (e.g. buttons) presented by software and provides requirements and recommendations on when and how to use
them. It includes a list of generic visual user-interface elements, as well.

Part 171: Guidance on software accessibility

Part 171 covers issues associated with designing for people with physical, sensory and cognitive disabilities, including those who are temporarily disabled, and the elderly.

Part 210: Human-centred design for interactive systems

Provides requirements and recommendations for human-centred design principles throughout the iterations of interactive systems.
Important to know

It is important not to confuse standards with best practices! For each product, the standard might vary from the most optimal solution. It is always recommended to perform user tests to find the solution that fits the specific problem.

Getting copies of the standards are, for an individual designer especially, fairly expensive. However, if a designer is interested, I would recommend to find a local library and see if they can make them available. Universities might have a copy of the standards available as well.