The gerund (gerundio) is a grammatical form that refers to actions in progress.  In English, we form gerunds with -ing: joking, laughing, dancing, playing.  In Spanish, however, we form them differently.
Starting from the infinitive, we remove the last two letters.  If these letters are -ar, we add -ando.  If these letters are -er or -ir, we add -iendo.  Hablando (hablar), comiendo (comer), viviendo (vivir).
In English, we use the gerund as a noun: Running is good for your health.  In Spanish, we can’t do this, and if we want to make a noun from a verb, we have to use an infinitive: Correr es bueno para la salud.

Since gerunds refer to actions in progress, they can’t be used alone.  Instead, they need to occur with another verb.  Most often, this verb is estar, but they can also be used with seguir, continuar, ir,venir, and andar.

The differences between these helping verbs are subtle, so a few examples will (hopefully) help explain.

Estoy buscando trabajo        I am looking for work
This is the most ordinary usage.

Sigo buscando trabajo        I am still looking for work, I keep on looking for work
Seguir and continuar have the same exact meaning when followed by a gerund: something is still happening, continues to happen, or keeps on happening.
Ando buscando trabajo        I go around looking for work; I am always looking for work
While andar means to walk, before a gerund it often implies a pointless or disorganized way of doing something rather than a more literal meaning.  It can also imply that something is habitual


The Gerund can be also used as an adverb for example

Voy haciendo mi tarea        I am doing my homework as I go (e.g. in the car)
Ir with a gerund tends to have a more literal meaning of doing something on the go.  You can ir comiendo, which suggests eating a sandwich on the highway, for example.  In a figurative sense, ir, unlike andar, implies a much more positive attitude on the part of the speaker.  Indeed, it generally implies progress.  If you were to say, ando aprendiendo a cocinar, I might get the impression that every few weeks you learn something, but if you tell me, voy aprendiendo a cocinar, I would assume that you were already making substantial progress.  (I might even accept your invitation to dinner.)

Viene diciendo tonterías        He arrives saying stupid nonsense;
                    He’s (still) saying (the same) stupid nonsense.
Venir, like ir, often implies physical movement, although towards the speaker.  When venir is used with a gerund in a figurative sense, it typically implies that something has been going on a long time, and perhaps even frustration that it has not changed.

Given all of this new information, I have a question for you: ¿estás aprendiendo español?  y ¿vas progresando?