It's easiest to understand the reintegration process as West Germany (The Federal German Republic, or BRD), annexing East Germany, (the German Democratic Republic, or DDR), as this is what happened.

After the Berlin Wall opened in 1989 (which was due to one security guard not knowing what to do after the leader of East Germany botched a press conference), the East German state steadily weakened, especially as soviet support stopped. East and West Germany first signed a treaty supporting economic and social reunification (eliminate border checks, common market) which lead to a later treaty of reunification. Under the reunification treaty East Germany was dissolved, and all it's lands joined West Germany, and became subject to West German laws and treaties. There was now only the Federal Republic of Germany (which is the official name of the Germany we know today). All of this was also ratified by the WWII Allies in a treaty as well, the Final Settlement in Respect to Germany and the German People.

For the first few years there were a lot of administrative hurdles, such as establishing bureaucracies and everything else required to run society effectively. Even though many East German government institutions existed, they were replaced or transformed to mirror the West German institutions which were now required.

What's important to note about East Germany is that everyone in that country knew at least a bit about West Germany and western culture, they are linguistically the same and all but a few corners (the island of Ruegen) received West German radio and TV signals. While East Germans did experience some shock in the first few years, these countries had only been separate for about 40 years, which really isn't that long.

Much of the economic development of the eastern German (or "new") states has been paid for by a "solidarity tax" on the much richer western states, which has inspired a bit of political animosity. Even though there has been much economic development the Eastern German states are still much poorer on average, but the living costs are significantly lower. The new states also have a demographic problem, as many young people left in the last 20 years to get higher paying jobs in the west or larger eastern cities (Berlin, Leipzig), as a consequence many small towns look like they might die out.

In the early years, the main changes someone in East Germany would have noticed in the early years were:

• New passports, new regulations, names of government departments changed.
• You could now go anywhere you wanted without a problem. Most people couldn't afford to travel the world, but it was a novelty to go to cities like Munich or Hamburg which you had only ever heard about, and which were very important in German history.
• There was a lot more selection in stores, and you had way more choice as a consumer. Once infrastructure was established (which happened very quickly) you could get goods from all over the world, which made people happy. The availability of fruit in supermarkets was a big thing, as the old East German supermarkets only had stuff seasonally.
• People could suddenly read, listen to, and watch a much greater variety of things, DDR TV and Radio were campy, and kind of sucked.
• Brown people showed up. In East Germany they had known of some foreigners, but they didn't really exist beyond people from other USSR nations. Seeing different races shocked a lot of people, and to this day the eastern German states have a larger problem with racism.
• You got to vote in real elections, this happened once before the DDR joined the BRD, and then everyone voted for representatives in the parliament in Bonn. Eventually the parliament moved back to Berlin.

For West Germans:

• Germany was reunited, which was nice, but you had to now pay a solidarity tax, which people complained about.