Speaking in tongues is referenced in both Acts 2 and 1 Corinthians 14. Are they expressions of the same gift? What do they share in common, and what is different between them?

Acts 2 begins with the Apostles being baptized by the Holy Spirit.

When the day of Pentecost arrived, they were all together in one place. And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting. And divided tongues as of fire appeared to them and rested on each one of them.

Acts 2:1-3

After baptism by the Holy Spirit they begin to speak in tongues:

And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance.

Acts 2:4

Acts 2 takes place within Pentecost, also known as the Feast of Weeks. Pentecost takes place 50 days after Passover and Jewish men would travel from afar to Jerusalem with sacrifices.

Now there were dwelling in Jerusalem Jews, devout men from every nation under heaven.

Acts 2:5

And all of these men were hearing the apostles speak in their native tongues and were puzzled by it:

And at this sound the multitude came together, and they were bewildered, because each one was hearing them speak in his own language. And they were amazed and astonished, saying, “Are not all these who are speaking Galileans? And how is it that we hear, each of us in his own native language?

Acts 2:6-8

The apostles were speaking in known languages such as from Mesopotamia, Egypt, Asia, Libya, Crete, and Arabia (see Acts 2:9-11) yet they were Galileans who are assumed to have not known those languages (Acts 2:7).

we hear them telling in our own tongues the mighty works of God.

Acts 2:11

Because it was a known language the audience could hear and understand the language naturally.

Apostle Paul mentions tongues and interpretation of tongues as spiritual gifts in 1 Corinthians 12:

For to one is given through the Spirit ... to another various kinds of tongues, to another the interpretation of tongues.

1 Corinthians 12:8, 10

The interpretation of tongues did not seem to be required by the hearers of tongues on the day of Pentecost because they were speaking in a known language (see Acts 2:11), or at least it is not called out specifically within the scripture.

Paul continues on about tongues in chapter 14:

For one who speaks in a tongue speaks not to men but to God; for no one understands him, but he utters mysteries in the Spirit. 

1 Corinthians 14:2

Speaks not to men but to God? No one understands him? This is completely different than the story of Pentecost, where the apostles spoke to other men and who understood them.

1 Corinthians 14:2 would seem to point to a different type of tongue or different expression of that gift, whereby the language being spoken is not readily understood by human ears.