A number of reasons are given for commemorating Lag B’Omer:
1. It commemorates the students of Rav Akiva who ceased dying during this day (Shelah, Pesachim 525).
2. This day is the yahrtzeit of Rabi Shimon bar Yochai, who revealed the inner secrets of the Torah (Chayei Adam Mo’adim 131:11).
3. This is the day that Rav Akiva granted ordination to his five students — among them Rabi Shimon bar Yochai (Pri Chadash OC 493).

Haircuts:
According to the opinion of the Shulchan Aruch one may not take haircuts or get married until the thirty-fourth day of the omer in the morning. The Rama feels that one may get a haircut on Lag B’Omer. The Rama explains that one must wait for Lag B’Omer morning (after Neitz Hachama) to get a haircut. According to the Rama one would not be allowed to get a haircut on the night of Lag B’Omer. However, the Mishnah Berurah cites authorities who permit getting a haircut on the night of Lag B’Omer. Harav Shlomo Zalman Auerbach zt”l is cited as explaining that the second view feels that since there are bonfires and parties that take place tonight one would likewise be permitted to shave. For normative halacha, a rav should be consulted.

As noted in a previous post, according to the Arizal shaving is not allowed even on Lag B’Omer.

Counting Sefira:

2) Harav Yitzchak Yosef shlit”a cites the above ruling and he adds that if one forgot to count the Omer the entire night and day of Lag B‘Omer, however, he mentioned in passing that “today is Lag B‘Omer” (e.g. he corrected the chazan who began to recite Tachanun that “today is LagB‘Omer“) he may continue to recite Sefira the following nights with a bracha. Post facto we can consider the statement of “Today is Lag B‘Omer” as a fulfillment of counting the Omer to allow him to continue to count with a bracha on the following nights.

Kevarim:

1) It is well known that there is a custom to go to the Kever of Rav Shimon Ben Yochai (or Rashbi) on Lag B‘Omer. Some poskim write that if one is in America and unable to go to the Kever Rashbi, he should try to daven at the grave of other tzaddikim. (Netai Gavriel Pesach vol. 3 page 342)