Oshimen = favorite member
Senbatsu: They have yearly senbatsu elections, popularity contests in July where fans vote for their favorite members. 2011 is the third year they've held the elections, but it's the first year when AKB48, SKE48, and NMB48 members were all up for contention, expanding the ballot to 152 candidates. Only the top 40 members were chosen.
One is eligible for voting when they are in the official AKB48 fan club and have the newest single. You get one vote per single, which is why there are some AKB crazies out there who buy boxes and boxes so that they have more votes.
The rank of a member determines how much face time, singing, and media exposure they receive.
Center: Top voted member. 2010 was Oshima Yuko, 2009 + 2011 was Atsuko Maeda "Acchan".
Media senbatsu: Top 12 members.
Senbatsu: Top 21 members.
Undergirls center: 22nd member.
Undergirls: 22nd to 40th members.
The ranks of CPM oshimen for this year's election. They all happen to be senbatsu:
#01 – Maeda Atsuko "Acchan" (139892 votes)
#02 – Oshima Yuko "Oshima" (122843 votes)
#04 – Shinoda Mariko "Shinoda" (60539 votes)
#05 – Watanabe Mayu "Mayuyu" (59118 votes)
#07 – Takahashi Minami "Takamina" (52790 votes)
#10 – Matsui Rena "Matsui" (36929 votes)
#11 – Miyazawa Sae "Miyazawa"(33500 votes)
#17 – Akimoto Sayaka "Akimoto" (17154 votes)
#20 – Masuda Yuka "Yuka" (14137 votes)
Hope this helped! If I made any mistakes no worries, Yuki88 will correct them.