A visa is an entry or residence permit issued by a country to a foreign citizen.
A visa is a legal formality that must be completed by any person travelling to a foreign country, regardless of the purpose of the trip.
Visas must be obtained at the consulates or embassies of the issuing country in person or, often, online. As long as the destination country allows for online applications, this is the best option.
You should find out about a country's entry laws before travelling, given that each country has specific immigration laws, which may vary depending on the nationality of the applicant. There are countries with free transit agreements for some travellers, in which no visa is required.
There are many types of visas, but they can be categorised into 2 types: short-stay visas and long-stay visas. Short-stay visas are typically requested for tourist, business and/or transit purposes. Normally, these visas allow travellers to stay in a country for up to 90 days (but note that there are countries with stricter conditions). Those who wish to reside, work or study should therefore apply for a long-stay visa.