Why are some of them "funcName()" and some of them "funcName = () =>"?

import React, { Component, PropTypes } from 'react';

export default class Stopwatch extends Component {
    state = {
      running: false,
      previouseTime: 0,
      elapsedTime: 0,
    }

    componentDidMount() {
      this.interval = setInterval(this.onTick);
    }

    componentWillUnmount() {
      clearInterval(this.interval);
    }

    onStart = () => {
      this.setState({
        running: true,
        previousTime: Date.now(),
      });
    }

    onStop = () => {
      this.setState({
        running: false,
      });
    }

    onReset = () => {
      this.setState({
        elapsedTime: 0,
        previousTime: Date.now(),
      });
    }

    onTick = () => {
      if (this.state.running) {
        var now = Date.now();
        this.setState({
          elapsedTime: this.state.elapsedTime + (now - this.state.previousTime),
          previousTime: Date.now(),
        });
      }
    }

    render() {
    var seconds = Math.floor(this.state.elapsedTime / 1000);
    return (
      <div className="stopwatch" >
        <h2>Stopwatch</h2>
        <div className="stopwatch-time"> {seconds} </div>
        { this.state.running ?
          <button onClick={this.onStop}>Stop</button>
          :
          <button onClick={this.onStart}>Start</button>
        }
        <button onClick={this.onReset}>Reset</button>
      </div>
    )
  }
}