Part one: the group as a whole
Similarities and differences between countries are largely a matter of perspective. What strikes most western lay observers is how different from the west are the forms of political and economic organization in eastern Europe, yet how similar they are as a group. They are all to a greater or lesser degree centrally planned. They all became communist dominated at, or soon after, the end of the Second World War. Except for Yugoslavia and Albania they are members of the same trading and military blocs, the Council for Mutual Economic Assistance (CMEA or COMECON), and the Warsaw Pact, which were set up in 1949 and 1955 respectively. CMEA was formed as a response to Marshall Aid, the offer of which initially included also the east European countries, but the USSR chose to establish its own organization instead. Many countries in this group are also still highly secretive with economic data and those which they produce often have to be treated with some scepticism.
In contrast, to those more familiar with eastern Europe, what stands out is the diversity within the group. The case of Yugoslavia is the most obvious. In the Second World War, Tito had led the largest resistance movement in Europe and its genuinely national roots, together with the absence of Soviet armies in the country at the time of liberation or after, were powerful factors contributing to Yugoslavia's ability to break away in 1948, and to pursue its own 'associational socialism' as compared to the 'state socialism' of the remainder. However, other countries too tried to achieve varying degrees of independence from the Soviet Union (East Germany in 1953, Hungary in 1956, and Czechoslovakia in 1968), but all these attempts were suppressed by the Soviet army. Only Poland's attempts at emancipation, in 1956 and then more seriously in the early 1980s, were allowed to be checked by internal forces.
There are also significant economic differences within the whole group.