Basically in this context (when you're making a suggestion) there's no difference.Пойдём can just be considered as a bit more persistent offer)

1. The first one means absolutely the same if you use them in the meaning : let's go. Off we go.

You just say: Пошли! Or Пойдем!

2. In the second case, there is a difference as these two verbs are used in different tenses.

•Пошли is used in the past and sometimes in the present: мы пошли в парк. We went to the park/we've just gone to the park/we're going to the park (we've moved/started to go already in the direction to the park)

•Пойдем is used as a future tense: Мы пойдем в парк.

We will go to the park/ Tomorrow we are going to the park

That's it:)

Мы пойдем в парк через час. - We'll go to the Park in an hour.    future time

Они пошли в парк утром. - They went to the Park in the morning.  past time

"Let's go to the park" = "Пойдем в парк".     future time

"Пошли" используется в отношении уже совершенного действия.

"Пойдем" используется в отношении действия, которое планируете совершить.