Polish belongs to a group of Slavic languages. It is a big group divided into three main branches: Western, Eastern and Southern. In Western, together with Polish, there are Czech and Slovak languages, Eastern: Russian, Ukrainian and Belarussian. Southern is another time divided into Western: Serbian, Croatian and Eastern Bulgarian and Macedonian.
Those languages generally are very similar. But there is a big difference: script - Latin in the Western group and Cyrillic in Eastern. It is connected with religion. There was a more significant influence of the Latin (Roman-Catholic) church in Western countries and Orthodox in Eastern. In the Orthodox church, the Church Slavonic language is used.
Slavic languages are similar, but it does not mean that we can understand each other. Basic words, for example, water is woda in Polish вода [voda] in Russian voda in Chech.
In 966 Mieszko I was baptized. Then the Latin alphabet was adopted to the Polish language. Before, probably it was only spoken language. The first written Polish words are in a Book of Henryków from 1270, and it is: "Daj uć ja pobrusza a ty pociwaj" what means "come let me grind a you take rest".
In Polish, there is a free word order, which makes it easier. Then it is getting more complicated. There are three genders in Polish: masculine, feminine and neuter. The plural is masculine and non-masculine. There are seven cases. But it sounds not too complicated; there are many exceptions from rules. Polish is a language of a lot of rules and much more exceptions.
There are also some difficulties with pronunciation. For example, Wawel spelled by W is pronounced as V. Some characters in Polish are pronounced in a different way than in English. Good example is and very tricky name of the city is Wrocław /ˈvrɒtswɑːf/ .
In Polish, not all letters are used, for example, no q, v and x. X is used only in one word and it is 'taxi'. There are some extra letters accents like ć, ś, ź, ń, ó with dots: ż, or with a little curve line in the down part: ą and ę, which can be hard for pronunciation because those are nose sounds.
Consonants in Polish also cause some problems. Sometimes there are clusters of 4 or even 5 together. I. E. Źdźbło - blade of grass or wstrząs - shock. In those words there are digrafs like sz, cz, rz, dz but there are even trigraphs: dzi. It looks like there are not enough vowels. In real, it is easy because one vowel mostly is one syllable.