The Ukrainian legislation defines both visa and visa-free entry into the country for foreign citizens and stateless persons. You did not specify which state you are a citizen of, so let's give you some general information.

Visa-free entry and transit through Ukraine is extended to citizens of the European Union member states (the Czech Republic is a member of the European Union since 2004), CIS countries, the United States, Canada and several others. To visit Ukraine, it will be enough to have a passport (if there is such a concept in the country), or a civil passport (for citizens of the Russian Federation, as well as countries where a single passport is valid).

The visa-free regime does not apply to immigration entry, visiting the country for training, or working in diplomatic missions and consulates in Ukraine, or for other purposes if the period of stay in Ukraine exceeds 90 days.