According to SEMPO head terms are:
Search terms that are short, popular and straightforward; e.g., “helicopter skiing.” These short terms are called “head terms” based on a bell-curve distribution of keyword usage that displays the high numbers of most-used terms at the “head” end of the bell curve graph. See also Tail Terms.
At the other end of the spectrum, tail terms are:
Search terms that are very specific, long phrases that include one or more modifiers, such as “cheapest helicopter skiing near Banff BC.” These longer, more specific terms are called “tail terms” based on a bell-curve distribution of keyword usage that displays the low numbers of little-used terms at the “tail” end of the bell curve graph. (See “The Long Tail” by Wired editor Chris Anderson.) Although long, specific and lesser-used tail terms have low CTRs, they are less competitive (and therefore cheaper) and often catch buyers at the end of the purchase decision process. This means that, even with low click-through numbers, tail terms can have good conversion rates. See also Head Terms.
In the context of law firm web marketing, head terms are terms like lawyer, injury attorney, or chicago divorce law firm. Examples of tail terms might include how do i find a lawyer that is right for me, what kind of lawyers handle work injuries, or who are the best divorce lawyers in chicago.
Head terms tend to have higher search volume, and thus, are typically more competitive (i.e. more pages competing for visibility for those terms).
Tail terms tend to have lower search volumes on a per phrase basis, but also tend to be much less competitive. These terms also tend to be much more difficult to anticipate.
Many search engine optimization consultants tend to sell clients on their ability to generate rankings for head terms. However, this isn’t always the most effective strategy.
Since they are more competitive, earning rankings for head terms is likely to take much longer than for tail terms. While these terms may be extremely targeted to your practice, and more likely to convert into potential client inquiries, when compared against an aggregation of relevant search terms, these terms actually tend to represent a smaller segment of total search traffic.
Instead of focusing all your resources on trying to organically rank for head terms, you may achieve more immediate results focusing your effort on longer tail searches and paid search for head terms.
In practice, this means considering tail terms for use in page/post titles and head terms as part of a paid search advertising like campaign, like adwords.