Longtail keywords are searches which are very specific in nature and tend to be long (think 4 words and more). Usually, long tail keywords work at a better conversion ratio than the short tail ones although the scale from one long tail keyword will be low (number of clicks and conversions). One can target these keywords by adding them to the account as an individual keyword or as a phrase or broad match of a short tail keyword.
If you decide to target these keywords via phrase or broad match of a short tail keyword, the bidding will be sub-optimal. The phrase keyword bid will be an averaged out bid of many long tail keywords which might not convert at the same rate. The correct way of bidding will identify the conversion ratio of each long tail keyword, add them to a new ad group and bid the appropriate value.
I do have a rule of thumb while creating new ad groups for long tail keywords. I ensure that the number of impressions for a long tail keyword needs to be higher than 50. 50 impressions give you a good scope of getting some clicks and hopefully some conversions. This number, however, varies with account to account as some advertisers would be in a niche where getting 20 impressions might be hard or some advertiser has 100s impressions from a lot of long tail keywords