user323: A follow up from a comment on this answer.
What are seednodes, and how do they differ from fallback nodes?

user153: DNS seeds aren't necessarily "seednodes" as they don't necessarily have have Bitcoin running as well.  Instead these hosts have standard DNS servers with A records that new Bitcoin nodes use for bootstrapping to learn of peers.
Currently, Bitcoin master shows these four:

bitseed.xf2.org
dnsseed.bluematt.me
seed.bitcoin.sipa.be
dnsseed.bitcoin.dashjr.org

Source code:
https://github.com/bitcoin/bitcoin/blob/master/src/net.cpp#L1170
// DNS seeds
// Each pair gives a source name and a seed name.

// The first name is used as information source for addrman.
// The second name should resolve to a list of seed addresses.


static const char *strMainNetDNSSeed[][2] = {
    {"bitcoin.sipa.be", "seed.bitcoin.sipa.be"},
    {"bluematt.me", "dnsseed.bluematt.me"},
    {"dashjr.org", "dnsseed.bitcoin.dashjr.org"},
    {"xf2.org", "bitseed.xf2.org"},
    {NULL, NULL}
};


user403: "Fallback nodes" is a page on the Bitcoin Wiki which lists some reliable Bitcoin nodes. "DNS Seeds" are DNS servers hard-coded into Bitcoin which return IP addresses of Bitcoin nodes which can be used for bootstrapping. "Seednodes" are Bitcoin nodes which have their IP addresses hard-coded into the Bitcoin client (see list here); they're used as an alternative bootstrapping method.

323: <p>A follow up from a comment on <a href="https://bitcoin.stackexchange.com/a/3709/323">this answer</a>.</p>

<p>What are seednodes, and how do they differ from fallback nodes?</p>
 153: <p>DNS seeds aren't necessarily "seednodes" as they don't necessarily have have Bitcoin running as well.  Instead these hosts have standard DNS servers with A records that new Bitcoin nodes use for bootstrapping to learn of peers.</p>

<p>Currently, Bitcoin master shows these four:</p>

<ul>
<li>bitseed.xf2.org</li>
<li>dnsseed.bluematt.me</li>
<li>seed.bitcoin.sipa.be</li>
<li>dnsseed.bitcoin.dashjr.org</li>
</ul>

<p>Source code:</p>

<p><a href="https://github.com/bitcoin/bitcoin/blob/master/src/net.cpp#L1170" rel="nofollow">https://github.com/bitcoin/bitcoin/blob/master/src/net.cpp#L1170</a></p>

<pre><code>// DNS seeds
// Each pair gives a source name and a seed name.

// The first name is used as information source for addrman.
// The second name should resolve to a list of seed addresses.


static const char *strMainNetDNSSeed[][2] = {
    {"bitcoin.sipa.be", "seed.bitcoin.sipa.be"},
    {"bluematt.me", "dnsseed.bluematt.me"},
    {"dashjr.org", "dnsseed.bitcoin.dashjr.org"},
    {"xf2.org", "bitseed.xf2.org"},
    {NULL, NULL}
};
</code></pre>
 403: <p>"Fallback nodes" is a page on the Bitcoin Wiki which lists some reliable Bitcoin nodes. "DNS Seeds" are DNS servers hard-coded into Bitcoin which return IP addresses of Bitcoin nodes which can be used for bootstrapping. "Seednodes" are Bitcoin nodes which have their IP addresses hard-coded into the Bitcoin client (<a href="https://github.com/bitcoin/bitcoin/blob/863e995b79ec388bf292d80f181912d01e20e2e5/src/net.cpp#L1198" rel="nofollow">see list here</a>); they're used as an alternative bootstrapping method.</p>
