1) What are these Segwit Addresses?
- There are 2 different Segwit address formats. They are P2SH (starting with a "3") and bech32 (starting "bc1")
- P2SH can be sent to by people using older Bitcoin software with no Segwit support. This supports backwards compatibility
- People sending from newer Bitcoin software that has Segwit and bech32 can send to the new address type that starts "bc1"
- People sending from older Bitcoin software that has no Segwit probably cannot send to the new address type that starts "bc1"
So, if people send you BTC from old software, give them the Segwit addresses that start with "3". If people send you BTC using new Segwit Bitcoin software, give them the Segwit addresses that start with "bc1".
If you don't know if someone has the newer bech32 software or not, there's no need to worry, their software will simply refuse to send the transaction, so no harm can be done.
2) How are these different from normal Addresses?
- You won't get the Segwit fee reduction unless you spend from one of these addresses
- The addresses starting "3" are normal addresses. They already exist in the Bitcoin blockchain now. The difference is that the Bitcoin script permits a wide variety of options, in this case, using Segwit in a way that old Bitcoin software can handle (obviously old software has no clue what Segwit is, it didn't exist back then, this method works around that)
- The addresses starting "bc1" are not normal addresses. They are brand new, and only work with Segwit
- With addresses starting "bc1", it doesn't matter if you USE UPPER CASE. or if you use lower case. bc1bech32segwitaddressescanbeupperorlowercase is the same address as BC1BECH32SEGWITADDRESSESCANBEUPPERORLOWERCASE, neither your wallet software or the blockchain will care
3) How are the fees low compared to other addresses?
There are no guarantees. Bitcoin isn't like going to Walmart (where a price is the only price you can get), it's like going to an auction house. The miners control whose transactions they include in blocks, and they can do it any way they like, even totally crazy ways that lose them money.
But, you could expect to pay fees of ~50% less than non-Segwit transactions, for a regular transaction where you pay from just one address to one address (with one change address). Why? Sort of complicated, but I can explain if you like.
4) If i upgrade from 2.9.3 to 3.0 will i have old addresses or it will swipe them?
No. Old addresses can exist alongside Segwit addresses in the same wallet.
If you're nervous about this, just use a new wallet for all Segwit transactions. But Electrum's approach to wallet specs is flexible enough to allow it, and also the standard way of doing it, it has alot of fail safety built into the design. It should be fine (Disclaimer: I don't use Electrum wallet)