Attaches to the -masu stem of a verb. Describes two actions occurring simultaneously, often with a literary or formal tone. Also used in the pattern つつある to indicate an ongoing change or process.
See also: つつある
彼は働きつつ学校に通っている。
He attends school while working.
Often used in the pattern つつも. Expresses a contrast between an expected outcome and the actual situation, similar to ながらも but more literary.
See also: つつも
悪いと知りつつ、嘘をついてしまった。
Even though I knew it was wrong, I ended up telling a lie.
The situation is gradually improving.
状況は改善されつつある。
The situation is gradually improving.
彼は体調が悪いつつも、会議に出席した。
Despite feeling unwell, he attended the meeting.
悪いと知りつつも、つい嘘をついてしまった。
Even though I knew it was wrong, I ended up lying anyway.
He was talking on the phone while walking.
ながら is the everyday, neutral way to say 'while doing something', whereas つつ is more literary/formal and also appears in the つつある pattern for describing an ongoing change, which ながら does not have.
The historical derivation is uncertain. It is a classical Japanese particle that has survived into modern formal and written usage.