From ~17th century until the very 20th century Lithuanians and Poles were literally a single nation. The same person would even use two names for himself in each language. For example, a famous interwar lawyer used “Michał Römer” and “Mykolas Römeris” respectively.
The situation of Polish language was similar to that of English in Ireland. After Poland and Lithuania signed the Union of Lublin (1569) and became a single country, the Polish language gradually became the one favored by ruling nobility. Eventually, it displaced Lithuanian language from more and more areas as Lithuanian was regarded to be the language of the lower classes. Capital city Vilnius and its surroundings as well as the manors in many places of Lithuania (except for Samogitia) switched to Polish over generations.
All this “Gente lituanus natione polonus” (Lithuanian tribe, Polish nation) way of thought came to a halt with Lithuanian national revival and the World War 1, after which Poland and Lithuania became separate entities. Still however the first Polish president Narutowicz was actually a brother of a signatory of Lithuania’s Declaration of Independence better known as Narutavičius, while the Polish dictator Józef Piłsudski regarded himself to be Lithuanian. Piłsudski was born in Zalavas (a village near Švenčionys) and he ordered to bury his heart in Vilnius (Rasos cemetery).
The dispute of both countries over Vilnius region and the subsequent Soviet occupation which used “Divide and conquer” tactics proved to establish the final boundary between the nations. Few would call themselves to be “Polish speaking Lithuanians” today, with most such people now considering themselves to be Poles (despite their forefathers having been ethnic Lithuanians rather than migrants from Poland). Neutral ethnic identity "a local" (tutejszy), popular pre-WW2, also nearly disappeared.
Currently, the Polish community is largely concentrated in southeastern Lithuania (Vilnius environs) and maintains a rural way of life. Vilnius city, while predominantly Polish-speaking in the 19th and early 20th centuries, was somewhat Lituanized due to urbanization, but still hosts 100 000 Poles in its homes (19,39% of Vilnius population and a half of Lithuania's Polish community). Nearly all Catholic the Poles are the most religious ethnic community.
Traditionally (since ~17th century) there was much multilingualism and diglossia in Vilnius area and the manors. The language to use depended on circumstances and political views on the Polish-Lithuanian issue. This multilingualism used to be Polish-Lithuanian or (in some villages) Polish-Belarusian, but was displaced by Russian-Polish during the Soviet russification drive. With the restoration of independence (1990) the situation is changing once again.
Lithuania's Polish community took a great hit under Stalin's policies when some 200 000 primarily Polish people (as many as there are Poles in Lithuania today) were moved from Lithuania to settle the lands acquired by Poland from Germany in the World War 2, while others were deported to Siberia. However, some new Poles were brought into Lithuania from Belarus in the same era. After 1950 the share of the Polish community in Lithuania largely stabilized (currently 6,65%). Today it is the only minority of Lithuania to have a strong minority rights political party with Lithuanian Poles Electoral Action having the majority in the predominantly Polish municipalities and enjoying representation in both Lithuanian and European parliaments.
See also: Top 10 Polish sites in Lithuania