What differences are there between the Roman Catholic and Eastern Orthodox understanding of the sacrament of marriage? I read somewhere a couple of years ago that Eastern Orthodox believe marriages contracted on earth, remain in place in heaven (however it is impossible to get married once you arrive in heaven). Whereas Catholics believe that marriages are dissolved at death and everyone in heaven is single.

I also heard that Eastern Orthodox allow divorce, but I don't understand how that fits with the "eternal marriage that persists even after death" theology I described above.

Is my understanding of catholic and orthodox marriages correct? More generally, what is the difference between the Catholic and Orthodox understanding of marriage?
I've never heard or read anything about marriage similar to what you quote (i.e. that marriages contracted on earth remain in place in heaven).  I've looked through the Longer Catechism of Philaret, Elder Cleopas' Truth of Our Faith (Vol. 2), and Protopresbyter Michael Pomazanski's Orthodox Dogmatic Theology (3rd ed.) and haven't encountered such a teaching.  You can read the Service of Marriage online here - I don't think you will find any suggestion of what you are saying.

Within the Eastern Orthodox Church a marriage can be dissolved "chiefly when it has been defiled by adultery, or when it has been destroyed by conditions of life (for example, long absence of one spouse, without word."1.  Generally only a second marriage is permitted, although a third may be "tolerated only as a lesser evil to avoid a greater evil", as  taught by St. Basil the Great2 (also a Doctor of the Roman Catholic Church):


  We look on third marriages as disgraceful to the Church, but do not
  absolutely condemn them, as being better than a vague
  fornication.3




1 Orthodox Dogmatic Theology, p.307.
2 Ibid.
3 Canon L, Second Canonical Epistle