Spider arguments are passed in the crawl command using the -a option. For example:

scrapy crawl myspider -a category=electronics -a domain=system

Spiders can access arguments as attributes:

class MySpider(scrapy.Spider):
    name = 'myspider'

    def __init__(self, category='', **kwargs):
        self.start_urls = [f'http://www.example.com/{category}']  # py36
        super().__init__(**kwargs)  # python3

    def parse(self, response)
        self.log(self.domain)  # system

Taken from the Scrapy doc: http://doc.scrapy.org/en/latest/topics/spiders.html#spider-arguments

Update 2013 : Add second argument

Update 2015 : Adjust wording

Update 2016 : Use newer base class and add super, thanks @Birla

Update 2017 : Use Python3 super

# previously
super(MySpider, self).__init__(**kwargs)  # python2

Update 2018 : As @eLRuLL points out, spiders can access arguments as attributes