What is the difference between active noise cancelling and passive isolation?

Passive isolation is just the shape of the headphone: in-ear tips or over-ear pads physically block sound. Active noise cancelling (ANC) also listens to your surroundings through microphones and generates an opposing wave, so it works best on steady drone — a plane, a bus, ventilation. ANC does very little against sudden sounds such as speech.

How long do wireless earbuds really last on one charge?

In-ear buds usually play for 4–8 hours, and the case adds another two to four full charges. Over-ear headphones last 20–40 hours. High volume, ANC and cold weather all cut into that — in winter expect roughly a fifth less.

Can I use one pair with both my phone and my computer?

Yes, if the headphones support multipoint: they stay connected to two devices at once and switch by themselves when a call starts on the computer. Without multipoint you have to disconnect one device manually every time.