Personal knowledge base is a tool that helps you to gather, classify, store, search, retrieve and share your Personal Knowledge. What makes PKBs unique is that they contain knowledge that is important to you personally and not to the public or to an organization. In other words, it contains distilled knowledge about specific topics that are of interest to you.

The term "personal knowledge base" was coined back in the 80ies, but it came into prominence by the publications of the computer scientist Stephen Davies and his colleagues in 2005.

However, the earliest example of such system was the hypothetical "Memex" machine described by Vannevar Bush in his prophetic article "As We May Think" back in 1945 in the Atlantic Monthly. The individual was supposed to use the memex as an automatic personal filing system, making the memex "an enlarged intimate supplement to his memory".

In fact, one can argue that the first example of such system are Leonardo Da Vinci's notebooks, which he maintained meticulously and daily, making continual observations of the world around him.

A good PKB solution gives you the following benefits:

• Knowledge generation and formulation - a good personal knowledge management tool helps you remember thoughts and ideas, explore them, and gain new understanding and insight. A good PKB software empowers you to find and to make new connections between already collected knowledge items thus helping grow your knowledge.
• Knowledge capture - it enables you to capture knowledge before it elusively disappears. How many times you see something that you deem as important, but you do not store it on time and you just forget about it afterwards? And even when you bookmark an important web page, it may just disappear. Having it stored locally could be very helpful.
• Knowledge organization - having your knowledge organized in a meaningful way helps you tremendously in finding relevant bits when you need them.
• Knowledge management and retrieval - perhaps the most critical aspect of a personal knowledge base is the ability to store knowledge permanently and accessible, ready to be retrieved at any time later. Knowledge is hard to acquire, and it is very important to have it stored in a durable system. Think of it as your second brain.
• Integrating heterogeneous sources - our knowledge comes from a variety of sources. Thus, we need a tool that makes possible storing and organizing any type of knowledge in a single place. Having all your personal knowledge in a single place allows you to review it regularly and to remember it easier.
• Knowledge independence - currently, you may have access to a vast pool of organizational knowledge, but once you leave a company, it is no longer available to you. You can use a personal knowledge base software to keep the knowledge relevant to you and to your fields of expertise even when you are no longer part of that organization.

The basic building blocks of such system are called "knowledge elements". These elements differ in their granularity and content in the different personal knowledge systems, but each system uses them as a fundamental unit that the user generates, manipulates and rearranges in order to reflect his mental model. Knowledge items can contain a single phrase or concept, but they can also represent free text notes, or links to external sources.

These elements are structured in some way. It can be in a top-down hierarchy, categories, interlinked items, or a combination of these.

Finally, these knowledge elements can be free-text or they can have specific types and additional properties.

An ideal tool for personal knowledge management requires a certain set of features:

• Clean, minimalist look - the PKB app should be user-friendly, and the user interface should be familiar to you from other applications that you already use.
• Text editor - continuing with the previous feature, you should be looking for tools that offer visual text editing (also known as WYSIWYG). While some people prefer using just plain text or even mark-up languages like Markdown, we recommend that you use a text editor with rich formatting and pictures in order to make your knowledge items more expressive.
• Web clipping - having a strong web clipper functionality is a must for any personal knowledge base app. The Web is our main source of information, so we need a way to store links, snippets (with source link) or complete copies of relevant web pages.
• Bi-directional linking or backlinks - this is an indispensable feature that helps us quickly see relationships between our knowledge items and create new connections between them. It gives you the power of a personal wiki.
• Full-text search - an ideal PKB solution needs a powerful search function for finding your knowledge. If you are power user, look for solutions that offer advanced search syntax, where you can specify in which parts of the items to search and what kind of items to find.
• Metadata - if you want to give more structure to your knowledge base, your tool needs strong metadata and tagging capabilities (tags), which allow you to better classify your knowledge elements, making them easier for grouping and sorting.
• Portability - your work may require that you travel a lot and use different computers. Having your personal knowledge base content stored on a USB drive or shared on a network could be a lifesaver.