Here comes Bob.
Bob is coming here.
Would anyone please explain what is the difference between these?
The difference is based on the concept of deixis, which are words that require context of space, time, or person to fulfill their meaning. For example, "I went to the store" means something different if I say it vs. if you say it.
In particular, you have:
Here comes Bob. This is deictic in terms of time, which is "now", and space, which is generally "here" relative to the speaker and "there where Bob is" as the speaker sees Bob.
Bob is coming here. This is deictic in terms of space, which is "here" and time, which is the present continuous tense (aka present progressive tense). This may indicate that Bob is coming now or in some relatively near future. For example, one could say any of the following:
"Bob comes here." This is deictic in terms of space only. In regards to time, it just means that it's his habit to come here.
The first sentence is an exclamation. "Here", and "There" are used with the present simple to form exclamations. Examples: Look! Here comes Mary! or *There comes George!*Both your sentences mean the same thing ,actually , but your second sentence just states the action , "John is coming here", whereas the first one stresses it. "Here comes Bob!"( it can show surprise, relief, agony,etc. depending on the previous context. Example: "Why is Mike so late;" "we're going to miss the train!" "Oh,at last" "There he is!"