If you are looking at migrating an application, then what Wictor said is correct - package a deployment up as a WSP, and deploy to the other environments.
However if you are looking at is entirely content based, and you currently have in one environment and are moving it all to another, either doing a content database move or a backup and restore is more than likely going to be the way to go to avoid unnecessary complications and limits relating to site templates.
If you are looking at merging content from one environment to another then it becomes more complex, but there are lots of third party tools that can help with this process. One of the most popular (although expensive) is AvePoint's DocAve. If this kind of content migration is something you're looking at doing regularly, i'd recommend giving it a look. There are many other tools that also do similar things - take a look around, they shouldn't be hard to find.