What is the difference between a hard fork and a soft fork?
A hard fork means that anyone who doesn't upgrade is left behind.
It creates 2 chains (forks). Your coins are duplicated so you have them on both, the first set of them with the original client and the other set with the new client.
The speed of each chain depends on the number of miners who upgrade. If all upgrade, then the old chain will basically stop.
If none of the miners upgrade, then the new chain will stop (or basically never get started).
The expectation is that normally one or other fork will get most of the miners and the other will be abandoned due to being so slow.
I have heard at some places that when a hard fork happens, (few) miners get their operations blocked by a major number of miners who come to a decision to apply some different rules they agreed upon. Is it true? Don't know much about a soft fork.
BIP 91 means that miners must ignore blocks that don't vote for segwit. That means that miners who don't vote for segwit will have their blocks rejected and they won't get paid for mining the block.
Are BIP 148 and BIP 91 the same? Or is BIP 148 based on the rules of BIP 91 or may be made based on it? What is that bit 1 and bit 4?
BIP-148 is that some users have said that if miners don't reject blocks which vote against segwit, then they will follow the ones that do.
BIP-148 was a threat and BIP-91 means that miners do what they asked.
Segwit2X is miners saying that they will comply with BIP-148 in exchange for big blocks in 3 months.
Block weights? Are you talking about block sizes here? I mean current ongoing block size is 1 MB (correct me if I am wrong), right?
The "weight" of a block depends on its size. Some parts of the block are "heavier". Weight is the new way to measure the size of a block and is part of segwit. The witness part of segregated witness is "lighter".
I also want to ask a question here that if the fork part gets settled here and we don't see a fork atm, will there still be a possibility of a hardfork in the future if any such scalability issues arise?
Segwit2X says that there will be a fork in 3 months to make blocks bigger. The miners have agreed to do this.
It will probably be a drama then too. Some users will probably threaten not to follow the 2MB chain if miners go through with their big block plan.
If that happens with a lot of users and enough miners, then there will 2 versions of Bitcoin. They could end up with Big Blocks Bitcoin and Bitcoin Classic or something similar.
It is also possible that everyone will agree to a consensus and almost everyone agrees to upgrade.
If that happens the legacy chain will likely not have enough miner support to continue.
If THAT happens, then the users who want the legacy fork to continue might hard fork so that they can change the mining rules so they can get their chain going again.
That would give three forks Big Blocks Bitcoin, Bitcoin Classic (Stalled) and Bitcoin New Mining Rules.