Share and ShareReplay, they are mainly the same. Just for ShareReplay, the subscriber subscribe after event emitted can also get value, similar to ReplaySubject.

// simulate url change with subject
const routeEnd = new Subject();

// grab url and share with subscribers
const lastUrl = routeEnd.pipe(
  pluck('url'),
  share()
);

// initial subscriber required
const initialSubscriber = lastUrl.subscribe(console.log); // nothing

// simulate route change
routeEnd.next({data: {}, url: 'my-path'});

// nothing logged
const lateSubscriber = lastUrl.subscribe(console.log); // my-path

In the code, only lateSubscribe got the value. initialSubscribe didn't get the value.

import { Subject } from 'rxjs/Subject';
import { ReplaySubject } from 'rxjs/ReplaySubject';
import { pluck, share, shareReplay, tap } from 'rxjs/operators';

// simulate url change with subject
const routeEnd = new Subject<>();

// grab url and share with subscribers
const lastUrl = routeEnd.pipe(
  tap(_ => console.log('executed')),
  pluck('url'),
  // defaults to all values so we set it to just keep and replay last one
  shareReplay(1)
);

// requires initial subscription
const initialSubscriber = lastUrl.subscribe(console.log);

// simulate route change
// logged: 'executed', 'my-path'
routeEnd.next({data: {}, url: 'my-path'});

// logged: 'my-path'
const lateSubscriber = lastUrl.subscribe(console.log);

Both got the value.

Side effect only execute once

import { timer } from "rxjs";
import { tap, mapTo, shareReplay, share } from "rxjs/operators";

//emit value in 1s
const source = timer(1000);
//log side effect, emit result
const example = source.pipe(
  tap(() => console.log("***SIDE EFFECT***")),
  mapTo("***RESULT***"),
);

/*
  ***NOT SHARED, SIDE EFFECT WILL BE EXECUTED TWICE***
  output:
  "***SIDE EFFECT***"
  "***RESULT***"
  "***SIDE EFFECT***"
  "***RESULT***"
*/
const subscribe = example.subscribe((val) => console.log(val));
const subscribeTwo = example.subscribe((val) => console.log(val));

If we use share or shareReplay(1)

//share observable among subscribers
const sharedExample = example.pipe(share());
/*
  ***SHARED, SIDE EFFECT EXECUTED ONCE***
  output:
  "***SIDE EFFECT***"
  "***RESULT***"
  "***RESULT***"
*/

const subscribeThree = sharedExample.subscribe((val) => console.log(val));
const subscribeFour = sharedExample.subscribe((val) => console.log(val));

Side effect only execute once