What is the difference between "Tishm'tenah" and "Netashtah"?
Rashi #1: The former refers to leaving the land fallow during the Shemitah; the latter, to not eating its produce (grain) after the time of Biy'ur. 1
Rashi #2: The former is a prohibition against basic labor - plowing and planting; the latter, against fertilizing and weeding. 2
Ramban #1 and Rashbam: With reference to the previous Pasuk, the former is a prohibition against planting crops, the latter, against harvesting. 3
Ramban #2 (in B'har, 25:5, citing the Mechilta): We learn from this Pasuk that it will not do to gather the harvest and distribute it to the poor, but that one is obligated to allow the poor entry to collect it themselves - even to the point of making breaches in one's wall to give them access to the crops. 4
Seforno: Relinquishing debts 5 and declaring one's produce Hefker, respectively.
Targum Yonasan: Leaving the land fallow and declaring the fruit Hefker, respectively.
Each fruit in its time.
See Ramban's objection to both of Rashi's explanations.
And the Pasuk aptly continues "ve'Achlu Evyonei Amecha ... ' (Ramban). And it concludes "Kein Ta'aseh le'Karm'cha ... " - as it is the way of the Torah to group together grain, wine and oil (as in the second Parshah of the Shema), though in effect, the prohibition extends to all food products that grow from the ground (Rashbam)/
Ramban: Although the Chachamim negated the need to do this.
Why does the Torah mention together the poor and animals eating in Shemitah?
Moshav Zekenim: Just like animals need not tithe Peros Shemitah, also the poor. This (Shemitah) is only in Eretz Yisrael, but not in Chutz la'Aretz.