Osaka is the second-largest city in Japan with over 17 million people, and is divided to 24 "wards", of which the most notable ones are Kita, the new city center, and Minami, which is the traditional and cultural center of the city.
Many things are very different from other places in Japan, English language is scarce if not nonexistent, and the people are really distinguishable from other places in Japan.
The Kansai airport is a common entry to the city and Osaka is also served by a bus network and the famous Shinkansen (the bullet train). Public transport is as it is in most places in Japan: fast, efficient, reliable and crowded.
Hostels in Osaka are abundant and vary greatly in quality, convenience of location, and hospitality. Be prepared to the fact that many of the Osaka hostel owners will not be able to speak any English.
Examples of activities in Osaka include Kaiyukan (a massive aquarium), Sumo Grand Tournament (held in March), and Spa World (open 24 hours). Activities in Osaka tend to be fairly expensive, but still less than you would pay in Tokyo.
Eating at local restaurants is part of the experience of being Osaka. There are plenty of delicious local dishes to try out, such as Battera (block sushi) and Tacoyaki (dumplings with octopus). Eating out in Osaka is also pretty pricey, but it is definitely worth it to get out there and sample the local foods.
Although Osaka is considered dangerous by Japan standards, it's still a very safe travel destination, as are most places in Japan.