In string theory the NS-branes are
These are also called the F1-brane and the F5-brane, respectively, for fundamental brane .
To be distinguished from the D-branes in type II string theory and from the M-branes in 11-dimensional supergravity.
The fundamental string carries electric charge under the background gauge field called the Kalb-Ramond field/B-field.
The NS5-brane carries magnetic charge under this field.
Notice how this relates the dimensions. The B-field is a circle 2-bundle with connection hence has higher parallel transport over 2-dimensional surfaces: the -dimensional worldsheet of the string. Its EM dual in the ambient 10-dimensional supergravity spacetime is a circle 6-bundle with connection with higher parallel transport over the -dimensional worldvolume of the NS5-brane.
Table of branes appearing in supergravity/string theory (for classification see at brane scan).