Define nanobody. How is it different from others ?

Nanobodies are based on the variable domain of the camelid antibodies which, unlike the common antibody, are comprised only of the heavy chain. This makes them quite a bit smaller (15 kda) than their parental full length antibodies, or antibodies derived from chicken, rabbit or humans. They also do not require to be assembled from fragments of the heavy AND light chains such as the other antibody fragments, e.g Fabs or scFvs.

see also the answer of the speaker to a related question below: