The difference between a portable espresso maker and a portable coffee maker is in the way coffee is made. To be classified as an espresso, water must be forced through ground coffee at 8 to 10 bars, and crema should make up at least 10% of the beverage (5). While not all portable espresso makers will necessarily meet this strict Italian definition, they come pretty close.
Other travel coffee makers have no such requirements and the resulting beverage won’t have a crema. Good examples would be pour-over devices or French presses. The Aeropress, which uses lower pressure, is a bit of a hybrid.