You can use the word "overall" at the beginning or end of a sentence.
It's used when your general opinion about something is different from your opinion about some of the specific parts of it. For example:
Overall, I think we did a great job. There were some mistakes, of course, but nothing really major.
Here's another example. It means that you mostly like the camera:
The Canon T2i has some weak points, but overall it's a great camera.
Notice that "overall" can be at the beginning of a sentence, or after a word like "but". It can also come at the end of a sentence:
I'm really happy with how it turned out overall.