How to create a web spider/crawler?

Overview: Web Crawlers work by parsing any particular web page and noting down if any hypertextlinks exist on that page that might be pointing to some other web pages then parsing those web pages and continuing in same way recursively and then indexing is decided for those web pages in sequence. This is a part of(rather an important part) of any search engine. It is an automated program that resides on a single computer and gets called whenever URL is typed in and passed to that program.

Creating a web spider/crawler[pseudo code]

Step # 1: Get the user's input -the starting url [optional: content type]
Step # 2: Get the URL of very first page and add it to a List containing URL s.
Step # 3: "While" List created in above step is not empty - Go to Step # 4 to Step #6 for each element in list.
Step # 4: Get the web page for that first URL and check if it is of same content type specified in Step # 1. If it is fine.
Step # 5: Check the same page if not in any blocked list [Optional]
Step # 6: If Step # 4 and Step # 5 are true =>Search the obtained web page for nay other hyper links. If any is there, Go to Step # 3. Add it to the list. and again continue recursively same steps in such a way for all URLs.

This is the basic flow of how a web crawler program should work. Other complications can be obviously added as per the needs and requirements.

Hoping was of some use. Feel free to correct( or add more information)

Thanks !