Example 1
Rebecca gave a moving, toilet-side eulogy for the deceased goldfish.
Our deepest sympathies to you, Rebecca. This somber sentence about a burial at sea is in active voice because Rebecca is the subject of the sentence and she's performing the sentence's action. If you wanted to reword it in the passive voice, it would look like this:
The deceased goldfish was given a moving, toilet-side eulogy by Rebecca.
Example 2
The Beatles were discovered by Brian Epstein.
The Beatles are the subject of this sentence, but they're on the receiving end of the sentence's action, which means this sentence is in passive voice. Because it's in passive voice, it puts the focus on The Beatles, which is something we're pretty sure John, Paul, George, and Ringo grew fairly accustomed to over the years (and rightly so).
If you were focusing on the possibly less-swoonworthy Brian, you might have said,
Brian Epstein discovered The Beatles.
Example 3
At the county fair, Clark set a new record for most hot dogs eaten in ninety seconds.
That's an impressive feat. Also impressive? The use of active voice in this sentence. Since Clark is the subject of the sentence, as well as the one performing the sentence's action, this is some textbook active voice.
If we wanted to put the focus on the record being set, we could switch this sentence to passive voice by making the record the subject of the sentence. It might look a little something like this:
At the county fair, a new record for most hot dogs eaten in ninety seconds was set by Clark, who will likely never look at a frankfurter the same way again.