A guide is a person who leads anyone through unknown or unmapped country. This includes a guide of the real world (such as someone who conducts travellers and tourists through a place of interest), as well as a person who leads someone to more abstract places (such as to knowledge or wisdom).

There are many variants of guides in this context, and guides are often employed in any aspect of travel or adventure, or wherever there is an advantage to the client in terms of knowledge provided in improving the overall travel experience or making the client feel more safe due to the presumed expertise of the guide.
These days guides will normally possess an area and field-specific qualification usually issued and/or recognised by the appropriate Guide's Association or licensing authority. However this is not always the case, and it is advisable for travellers paying a premium to go on an organised tour or journey because they think this will mean they will have a better experience to check the guide's qualifications beforehand.
Explorers in the past venturing into territory unknown by their own people invariably hired guides. American West explorers Lewis and Clark hired a Shoshone Indian woman Sacagawea, and Wilfred Thesiger hired guides in the deserts that he ventured into, such as Kuri on his journey to the Tibesti Mountains in 1938.
Aside from knowing the way or the area geographically, modern guides are usually hired to act as interpreters for those travellers who do not speak the local language and provide cultural, historical or other information on the area visited. Travel companies organising tours of large groups often have a guide or tour leader accompany the group. They might also be trained in First Aid and have other skills that reduce the risk for the tour operator to conduct these tours or the travel agency selling them.
Here are some examples of guide professions:

Antoni Gaudí i Cornet (Catalan pronunciation: [ənˈtɔni ɣəwˈði]; 25 June 1852–10 June 1926) was a Spanish Catalan architect and figurehead of Catalan Modernism. Gaudí's works reflect his highly individual and distinctive style and are largely concentrated in the Catalan capital of Barcelona, notably his magnum opus, the Sagrada Família.

Much of Gaudí's work was marked by his four life passions: architecture, nature, religion and love for Catalonia. Gaudí studied every detail of his creations, integrating into his architecture a series of crafts in which he was skilled: ceramics, stained glass, wrought ironwork forging and carpentry. He introduced new techniques in the treatment of materials, such as trencadís, made of waste ceramic pieces.

After a few years under the influence of neo-Gothic art and Oriental techniques, Gaudí became part of the Catalan Modernista movement which was reaching its peak in the late 19th and early 20th centuries. His work transcended mainstream Modernisme, culminating in an organic style inspired by nature. Gaudí rarely drew detailed plans of his works, instead preferring to create them as three-dimensional scale models and molding the details as he was conceiving them.