Slovak is a western Slavic language, closely related to Czech. The two are mutually comprehensible. For centuries Hungarian was the official language in much of the territory of modern-day Slovakia, though command of Hungarian these days is limited mainly to ethnic Hungarians in the south of the country.
English speakers are likely to find the language something of a challenge. English is not widely known outside the larger cities, though nearly anyone can muster a few words in a pinch. This shouldn't deter you. Most museums and restaurants and anywhere else you're likely to go will have English information on hand. Almost every tourist information center and hotel will have a least one English speaker. If you know some German, that may help as well.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.