Since Poland became a Schengen country, border controls between Poland and Germany, Slovakia, Lithuania, or the Czech Republic have been abolished. This means that, for short-term visits, you need only apply for a Schengen visa and are then more or less free to travel between participating countries. Schengen visas are valid for three months. However, in order to legally work in Poland, expats need to apply for a work permit in addition to their visa.
Work Visa

If you wish to live and work in Poland legally, you need to apply for a valid visa with the responsible authorities. In order to do so, you need to turn to the nearest Polish Consulate before moving to Poland. All in all, Polish consulates issue three different types of visas: Airport Transit Visas (A), Short-Stay (Schengen) Visas (C), and National Long-Stay Visas (D). Expats whose assignments exceed 90 days need to apply for a long-term visa, which also allows them to visit other Schengen countries.

In order to apply for a long-term visa, you need to submit the following documents:

a visa application form, duly completed
a passport, valid until at least three months after the visa expires
biometric, passport size picture
payment proof of visa fee
travel insurance or international medical insurance
proof of sufficient financial means
document stating the purpose of your visit
document stating the reason for a long-term stay

In addition to a regular long-stay visa, you will have to secure a work permit and submit it with your visa application. Please remember that it is not possible to apply for a work permit or a visa from within the country, and it is strictly prohibited to work in Poland without proper authorization.