Answer:
   
   Morning services are composed of 7 parts:
    1. The morning blessings. In this part we thank G-d for another day.
       Originally each blessing was said as you did that particular thing
       for the first time that day--gird your belt, tie your shoes, learn
       Torah, etc.. However today they are folded onto the begining of
       services.
    2. The order of sacrifices. The prophets tell us that someone who
       sincerely studies the laws of sacrifices gets as close as possible
       to offering one. So, we read the Mishnayos about the various
       offerings in order to gain some measure of atonement.
    3. Pesukei diZimrah (lit: verses of songs of praise). Some chapters
       of Psalms, bracketed by an opening and closing blessing. The main
       point of this part is to be a "warm-up", to get into the proper
       frame of mind, before the next three parts. If you get to services
       too late to say Pesukei diZimrah and still say the main prayers
       with the congregation, you should skip them. Or perhaps skip all
       but "Ashrei"--depending upon the time available. Most decisors
       opine that you should still say the ones you skipped some time
       during the day. The Vilna Gaon ruled that you should not. The
       debate is whether the section exists only as warm-up, or primarily
       as warm-up but also serves other purposes. As to whether someone
       who has a short attention span is best served using up all of
       one's attention on Pesukei diZimrah so that the later prayers
       become mindless is a question for that person's Rabbi. It's
       probably also related to where you stand on that debate.
       Those of us of the Sesame Street sound-bite generation should be
       working toward slowly building up that preparation time. Still,
       there are days where such a person should just say the opening
       blessing, Ashrei, the closing blessing, and then study Torah at
       their seat while waiting for the congregation to get up to Shema.
       The next three parts are three actual and distinct mitzvos.
    4. The Shema, with two blessings before and two after.
    5. The Amidah, the actual formal prayer.
    6. Tachanun, a framework in which one is supposed to insert informal
       prayers. In other words, the Amidah serves to remind man what he
       ought to consider important, and therefore what his relationship
       with G-d ought to look like. Tachanun has some of that, but it's
       more actually relating to G-d, turning to your Parent with what's
       on your mind. [Not that the masses actually remember that this is
       what Tachanun is for. In practice, it is far too often yet another
       formalized text with nothing personal interjected.]
    7. The closing. Most famously, this includes Aleinu.
       
   The afternoon service, coming in the middle of the workday, has only
   Ashrei as an intro, leading to the Amidah, Tachanun and Aleinu. People
   simply don't have the time for a longer service.
   
   The evening service is obligatory only because universal customs ought
   not be broken. It's not an obligation of the same magnitude of the
   other two, and therefore they started it with the Shema, with no
   warm-up.