The concept behind an eiruv is that the people making it join together as one large household. Each household contributes some food, which is put in a common area, and all are welcome to come and eat. The area within the eiruv must be enclosed, and there is much Talmudic discussion on what exactly constitutes an enclosure.
Human nature is to be forgetful, even in the midst of doing something. This human frailty is what leads the Mishnah (11a) to rule that a tailor may not go into the street with his needle nor may a scribe go out with his quill on Friday afternoons just before dark, lest they forget and accidentally carry their materials on Shabbat (in a place which has no eruv). Yet, the Gemarah (12a) rules that one may go out while wearing tefillin late on Friday afternoon, as “Rava bar Rav Hunah taught that one is obligated to feel his tefillin every moment and moment”.
Much of Masechet Brachot deals with non-legal matters - extolling the importance of prayers and blessings and recording many stories involving our great sages, to cite two main themes. While many of the laws impacting on our daily rituals are expounded upon, there is little of the intense and detailed argumentation over points of law we find in other places in the Talmud.
As we move to Masechet Shabbat, we immediately notice a shift to more technical aspects of halacha and legal argumentation, beginning with the intricate laws of carrying on Shabbat.