Spiders are also known as crawlers, every search engine has its own crawler. The crawler of Google is called GoogleBot. They are responsible for the complete process that includes crawling, indexing of websites, processing and retrieving of results in search engine result pages SERPs.
When you hear people talk about Crawlers in the context of SEO, they are referring to the programs that search engines use to scan and analyze websites in order to determine their importance, and thus their ranking in the results of internet searches for certain keywords.