Alexa provides a global and country specific site rank for a web site. For a country site rank, the site needs to have significant traffic from that country. Lot of people do research on competitors using Alexa rank. It can also be useful to to convince others (advertisers, etc.) about your approximate traffic and site popularity.
Alexa rank is based on the data sent by Alexa toolbar to Alexa servers. Assuming the sample of people who have installed Alexa toolbar is a good representative of all internet visitors, the traffic stats with Alexa should provide good relative rank. This is a big assumption and can easily lead to errors in stats. Here are some observations I had about Alexa rank during past few years:
- Alexa rank fluctuates a lot if you traffic is low. Tentatively for monthly visits > 3000, you should be able to see some meaningful rank.
- Global rank is more reliable than country specific ranks. I have seen less fluctuations and errors in global rank as compared to country specific rank.
- I think your rank may get impacted if your site has Alexa siteowner traffic widget. So if you compare two sites, you may want to keep this factor in mind.
- If you have installed Alexa toolbar yourself, this may increase you Alexa rank. This may impact low traffic sites more as the traffic from developer machine would be significant.
Alexa rank is pretty useful metric but it should be taken with a pinch of salt as same site can have different rank based on various factors. But in spite of all the error possibilities, it is an excellent way to get approximate traffic rank and relative stats of a web site.